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Equine clinical pharmacology
Equine clinical pharmacology
Equine clinical pharmacology
Equine clinical pharmacology
Equine clinical pharmacology
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Equine clinical pharmacology

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  • 1. SAUNDERSAn imprint of Elsevier Limited© 2004, Elsevier Limited. All rights reserved.No part of this publication may be reproduced, stored in a retrieval system,or transmitted in any form or by any means, electronic, mechanical,photocopying, recording or otherwise, without either the prior permission ofthe publishers or a licence permitting restricted copying in the UnitedKingdom issued by the Copyright Licensing Agency, 90 Tottenham CourtRoad, London WH 4LP. Permissions may be sought directly fromElseviers Health Sciences Rights Department in Philadelphia, USA: phone:(+1) 215 239 3804, fax: (+1) 215 239 3805, e-mail:healthpermissions@elsevier.com. You may also complete your request on-line via the Elsevier homepage (http://www.elsevier.com). by selectingCustomer Support and then Obtaining Permissions.First published 2004 Reprinted 2004, 2005ISBN 0 7020 2484 8British Library Cataloguing in Publication DataA catalogue record for this book is available from the British LibraryLibrary of Congress Cataloguing in Publication DataA catalogue record for this book is available from the Library of CongressNoteMedical knowledge is constantly changing. Standard safety precautionsmust be followed, but as new research and clinical experience broaden ourknowledge, changes in treatment and drug therapy may become necessaryor appropriate. Readers are advised to check the most current productinformation provided by the manufacturer of each drug to be administeredto verify the recommended dose, the method and duration of administration,and contraindications. It is the responsibility of the practitioner, relying onexperience and knowledge of the patient, to determine dosages and thebest treatment for each individual patient. Neither the Publisher nor theeditors assume any liability for any injury and/or damage to persons orproperty arising from this publication. The Publisher_ your source for books, journals and multimedia in the health scienceswww.elsevierhealth.com Working together to grow libraries in developing countries www.elsevier.com I www.bookaid.org I www.sabre.org The Publishers policy isto use paper manufactured fromsustainable forestsPrinted in China I
  • 2. ContributorsJoseph J. Bertone DVM MS Diplomate ACVIM Michel Levy DVM Diplomate ACVIMProfessor of Equine Medicine, Western University of Health Purdue University, School of Veterinary Medicine,Sciences, Pomona, CA, USA West Lafayette, IN, USAI. Mark Bowen BvetMed Cert VA Cert EM (IntMed) MRCVS Sandy Love BVMS PhD MRCVSRoyal Veterinary College, University of London, UK Department of Veterinary Clinical Studies, University of Glasgow, UKRobert M. Christley BVSc PhD Dipl VCS MVCS DiplomateECVPH MRVCS Jennifer M. MacLeay DVM PhD Diplomate ACVIMDepartment of Veterinary Clinical Science, Department of Clinical Sciences, Colorado State University,University of Liverpool, Neston, South Wirral, UK Fort Collins, CO, USAKevin T. T. Corley BSc BVM&S MS PhD Diplomate ACVIM & Celia M. Marr BVMS MVM PhD DEIM Diplomate ECEIM MRCVS Beaufort Cottage, Equine Hospital, Newmarket, UKACVECC MRCVSRoyal Veterinary College, University of London, UK Diane E. Mason DVM MS PhD Diplomate ACVA Department of Clinical Sciences, Kansas State University,Laurent Couetil DVM Diplomate ACVIM Manhattan, USAPurdue University, School of Veterinary Medicine,West Lafayette, IN, USA Andrew Matthews BVM&S PhD FRCVS McKenzie, Bryson and Marshall, Kilmarnock, UKKarina COX BSKL Maddy Equine Analytical Chemistry Laboratory, Michael J. Murray DVM MS Diplomate ACVIMSchool of Veterinary Medicine, University of California, Merial Ltd, Duluth, GA, USADavis, USA Bonnie R. Rush DVM MS Diplomate ACVIMPatricia M. Dowling DVM MS Diplomate ACVIM & ACVCP Department of Clinical Science, Kansas State University,Department of Veterinary Biomedical Sciences Manhattan, USAWestern College of Veterinary Medicine,Saskatoon, Canada Harold C. Schott II DVM PhD Diplomate ACVIM Department of Large Animal Clinical Sciences,Jonathan Elliott MA VetMB PhD Cert SAC Diplomate ECVPT Veterinary Medical Center, Michigan State University,MRCVS East Lansing, USARoyal Veterinary College, University of London, UK Janice E. Sojka VMD MS Diplomate ACVIMClara K. Fenger DVM PhD Diplomate ACVIM Purdue University, School of Veterinary Medicine,Equine Internal Medicine Consulting, Georgetown, KY, USA West Lafayette, IN, USALinda J. I. Horspool BVMS PhD DipECVPT MRCVS Sally Vivrette DVM PhD Diplomate ACVIMIntervet International, Boxmeer, Netherlands Triangle Equine Mobile Veterinary Services, Cary, NC, USACynthia Kollias-Baker DVM PhD Diplomate ACVCP Tom Yarbrough DVM Diplomate ACVSRacing Laboratory, College of Veterinary Medicine, School of Veterinary Medicine, University ofUniversity of Florida, Gainesville, USA California, Davis, USA
  • 3. PrefaceEquine clinical pharmacology is by definition The contributors to this volume, recognizedthe study of drugs in equine clinical cases. experts in their specialties, were selected on theRational drug therapy, founded on pharmacolog- basis of their particular research and/or clinicalical principles, requires a medical diagnosis. In specialization and expertise. Their recommenda-fact, without this diagnosis and an understand- tions for drug therapy are based on studiesing of the relevant pathophysiology, knowledge reported in veterinary medical literature, whereof pharmacologic principles and specific thera- possible, and on their own personal experience.peutic objectives, one cannot rationally prescribe The medications described do not necessarilydrug therapy. have specific approval from the European This first edition of Equine Clinical Pharmacology Medicines Evaluation Agency, Food and Drugsbrings together many of the topics pertinent to Administration or other similar agencies, for thedaily practice for equine veterinarians. This book treatment of the diseases for which they are rec-has a number of goals. It aims to provide both ommended. Whilst every effort has been made toveterinarians and veterinary students with a include the most up-to-date recommendations,source of current information about the rational information about drug dose rates and dosageuse of drugs, including best practice principles, in forms is constantly changing. The dose ratesthe treatment of specific diseases in the horse. given should generally be considered as averagesSecondly,it aims to provide an incentive for adopt- or starting points and thus require individual-ing a rational approach to drug therapy in clinical ization. Similarly, new drugs that were notcases and in addition to stimulating sustained approved at the time of publication may appearinterest in the conducting of controlled studies of on the market and other drugs or dosage formsthe safety and efficacy ofdrugs in diseased horses. may no longer be available. This book has been organized mainly around This book could not be all-inclusive. It isbodily systems, since many clinical problems in hoped, however, that it will stimulate the contin-horses are related to disorders of one or more of uation of interest and studies in equine clinicalthese systems. The editors have made an effort to pharmacology and help better arm the equineinclude information on pathophysiology and the veterinarian for the therapeutic decisions theydosage, best practice guidelines, precautions and make on a daily basis.potential adverse effects of the drugs discussed. Itis hoped that this information will help the reader Joseph J. Bertonein making risk/benefit assessments and in deter- Linda J.1. Horspoolmining the therapeutic objectives for each equinetreated.
  • 4. AcknowledgementsThis 1st Edition of Equine Clinical Pharmacology technical content and for providing many benefi-would not have been possible without the gener- cial suggestions. The manuscript was greatlyous support and contributions of many people. enhanced by their thoughtful attentions. ThanksThe contributors, selected because each is rec- also to Joyce Rodenhuis, Samantha Ross and Zoeognized as among the most accomplished in Youd at Elsevier Limited for their guidance andtheir respective disciplines, provided up-to-date support particularly during the final stages ofchapters discussing both modem trends and best preparation of the manuscript and for theirpractice principles and managed to adhere to encouragement in getting this project back onsometimes apparently impossible deadlines. track.This work would of course not exist without the Joseph J Bertone would like to dedicate thisexcellent and well focused contributions of: Mark work to Mel and the children, Tina and Carmine,Bowen, Rob Christley, Kevin Corley, Laurent and to his parents Tina and John.Couetil, Karina Cox, Patricia Dowling, Jonathan Linda Horspool would like to express her heart-Elliott, Clara Fenger, Cindy Kollias-Baker, felt thanks to her parents Billand Una Horspool forMichael Levy, Sandy Love, Diane Mason, Andy their continued encouragement and support andMatthews, Celia Marr, Michael Murray, [en to Marco Franken and his family, Wily,Patricia andMacLeay, Bonnie Rush, Harold Schott II, Janice Frank Franken, Kim Konings, and Francis and LolaSojka, Sally Vivrette and Tom Yarbrough. A spe- Kuenen, for their patience and understanding.cial thank you also to Michele Doucet and Jill She dedicates this work to the memory of TonnyPrice for critiquing chapters of the manuscript for Franken-Franssen.
  • 5. CHAPTER CONTENTSIntroduction 1Physicochemical factors in drugtransfer across membranes 2 Basic principles ofAbsorption 2 veterinary pharmacology Drug administration 3 for equine practitionersDistribution 4 Joseph J. BertoneBiotransformation 5Elimination 6 Urinary excretion 6 Gastrointestinal excretion 6 Excretion in milk 6Pharmacokinetics 7 Clearance 7 Half-life 7 INTRODUCTION Mean residence time 8 Volumesof distribution 8 Unfortunately, drugs do not distribute in living Bioavailability 10 organisms as though injected into balloons filledOther clinically relevant concepts 10 with fluid. The study of how drugs react in living The loading dose 10 tissues and organisms is pharmacology. The science Maximum plasma concentration 10 of pharmacology includes many different fields.References 10 The relationship between the dose of a drug given to an animal and the use of that drug in treatingSuggested reading 11 diseases is pharmacokinetics; what the body does to a drug and what a drug does in the body is pharmacodynamics, and whether it is desirable or undesirable is toxicology. Pharmacokinetics is the study of absorption, distribution, biotransformation (metabolism) and excretion of drugs. The end result is determined by the physical, chemical and biochemical prin- ciples that govern the transfer and distribution of drugs across biological membranes. These factors and the dosage (route, dose and frequency) deter- mine the drug concentration at the site of action and the intensity of a drugs effects as a function of time. Pharmacodynamics is the study of the mech- anisms of action of drugs and their biochemical and physiological effects. Toxicology is the field of pharmacology that deals with the adverse effects of drugs used in therapy and of other non-drug chemicals. A drugs usefulness as a therapeutic agent is critically dependent on its ability to produce desir- able effects and a tolerable level of undesirable effects. Therefore, the selectivity of a drugs effects is one of its most important characteristics. Drug therapy is based on the correlation of the effects of drugs with the physiological, biochemical,
  • 6. 2 EQUINE CLINICAL PHARMACOLOGYmicrobiological and/or immunological aspects transmembrane distribution of a weak acid orof the disease in question. Disease may modify base is determined by its acidic dissociationthe pharmacokinetic properties of drugs by alter- constant (pKa) and by the pH gradient across theing absorption into the systemic circulation and membrane. The proportions of drug in each statedrug disposition (distribution, metabolism and are calculated using the Henderson-Hasselbachelimination). In addition, sufficient concentrations equation. For a weak acid:of a drug must be present at its sites of action in pH = pKa + log(A - /HA)or?er to produce its characteristic effects. Clearlythis depends on the concentrations produced in where A-is the ionized drug and HA the union-the milieu around the cells in question, which is a ized drug. For a weak base:function of dosage, the extent and rates of absorp- pH = pKa + log(B/HB+)tion, distribution, biotransformation and excre-tion, and the rate of transfer of the drug across where B is the unionized drug and HB+ is thebiological membranes. ionized drug. Therefore, when the local pH is equal to the pKa of a drug, the drug will be 50% ionized and 50% unionized (log 1 = 0). There isPHYSICOCHEMICAL FACTORS further information on this area in the suggestedIN DRUG TRANSFER ACROSS reading.MEMBRANES Passive diffusion across cell membranes dom- inates the absorption and distribution of mostDrugs are transported in the aqueous phase of drugs. Drugs enter cells down concentrationblood plasma. To have an effect, drugs must enter gradients. This transfer is directly proportional tocells or reach cell membrane receptors. All aspects the magnitude of the concentration gradient acrossof the absorption, distribution, biotransformation the membrane and to the oil to water partitionand elimination of drugs involve transfer across coefficient of the drug (lipid solubility). Therefore,cell membranes. Other barriers, including multiple diffusion is fastest when there is a large partitionlayers of cells (e.g. the intestinal epithelium or coefficient and large concentration gradient acrossepidermis), also exist. the membrane. For ionized compounds, the equili- Biological membranes are essentially lipid; brium concentrations are dependent on the dif-consequently, the rate at which drug molecules ferences in pH across the membrane and the pKacross these barriers is dictated primarily by their of the drug. For unionized compounds, once thelipid solubility. Molecular size and shape and the equilibrium is reached, the concentration of freesolubility at the site of absorption also directly drug is the same on both sides of the membrane.affect the absorption of drugs. The other import- Active, selective mechanisms play an import-ant factor that determines the ability of a drug ant role in the absorption of some drugs. Energyto cross these biological barriers is the degree of is used to move drugs across membranes, oftenionization. against the concentration gradient. There is active Most drugs are weak acids or bases and are transport of some drugs in the kidneys (tubularpresent in solution as both the ionized and union- cells), liver and nervous system. Facilitated diffu-ized forms. Ionized molecules are usually unable sion is a carrier-mediated transport process thatto penetrate lipid cell membranes because they are requires no input of energy and that does nothydrophilic and poorly lipid soluble. Unionized occur against an electrochemical gradient.molecules are usually lipid soluble and can dif-fuse across cell membranes. "Like is unionized inlike", meaning that a weak acid will be most ABSORPTIONunionized in a fluid with an acidic pH and aweak base will be most unionized in a fluid Absorption is the rate and extent to which a drugwith a basic pH. Under most circumstances, the leaves its site of administration. Many variables
  • 7. 1. BASIC PRINCIPLES OF VETERINARY PHARMACOLOGY FOR EQUINE PRACTITIONERS 3affect the transport of drugs across membranes factors. The choice of the route of drug admin-and hence influence the absorption of drugs. istration is based on understanding these con-Absorption is dependent on drug solubility. Drugs ditions. Table 1.1 lists common routes of druggiven in aqueous solution are absorbed more administration.rapidly than those given in lipid solutionsbecause they mix more readily with the aqueousphase at the absorption site. Drugs in solid form DRUG ADMINISTRATIONmust first dissolve, and this may be the limiting Intravenous administrationfactor in absorption. Bioavailability indicates the extent to which a Problems with absorption can be circumventeddrug enters the systemic circulation. Factors that using intravenous (i.v.) administration of an aque-modify the absorption of a drug can change its ous solution of a drug. The desired concentrationbioavailability. A drug that is absorbed from the of a drug in blood is obtained with great accuracygastrointestinal tract enters the portal circulation and immediacy. The dosage can be titrated toand must first pass through the liver before it effect,such as in the induction of anesthesia, wherereaches the systemic circulation. Drugs that are the drug dose is not predetermined but adjustednot absorbed from the gastrointestinal tract, or to the animals response. In addition, irritantare metabolized extensively in the liver and/ or solutions can be given in this manner, since bloodexcreted in bile, will be inactivated or diverted vessel walls are relatively insensitive to drugbefore reaching the general circulation. If the meta- forms that are highly irritant when given by otherbolic or excretory capacity of the liver for a drug routes. The major liability of intravenous admin-is excessive, the bioavailability of the drug will istration is that adverse reactions are more likelybe substantially decreased (i.e.the first-pass effect). to occur since high concentrations of drug areReduced bioavailability not only is a function reached rapidly both in plasma and tissues.of the anatomical site of absorption but also is Repeated intravenous injections require an intactaffected by many physiological and pathological vein or, often, catheter placement. Table 1.1 Common routes of drug administration and their characteristics Route Absorption Use Precautions Intravenous None (directly into the Emergency use or largevolumes; Increased risk of adverse circulation); potentially Absorption is bypassed effects; inject solutions slowly immediate effects Not suitable for most lipid solutions or insoluble substances Subcutaneous Rapid if an aqueous Some less-soluble suspensions Not suitablefor large solution; less rapid if and implantation of solid pellets volumes; irritant substances depot formulations or depot forms cause pain and/or necrosis Intramuscular Rapid if an aqueous Moderate volumes; lipid vehicles; Inadvertent intravenous solution; less rapid if irritant drugs injection; pain or necrosisat depot forms injection site Oral Rate and extent are Maximize compliance (convenient); Absorption (bioavailability) highly variable and economical; usually safe may be erratic and incomplete dependent on many factors Pulmonary Rapid; local application Usually for the treatmentof Irritantsubstancesmust be for pulmonary disease pulmonary disease avoided; rapid absorption may induce high plasma concentrations and adverse effects
  • 8. 4 EQUINE CLINICAL PHARMACOLOGYIntramuscular administration absorption of drugs into the bloodstream, avoid- ance of the hepatic first-pass effect and localDrugs in aqueous solution are absorbed rapidly (topical) application of the drug in the case offollowing intramuscular (i.m.) injection, although pulmonary disease.this varies depending on factors such as the bloodflow to the injection site. In humans, absorptionfrom the deltoid or vastus lateralis muscles is Topical applicationfaster than from the gluteus maximus. Absorptionfrom this site is slower in females than in males. Drugs are applied topically primarily for localThis has been attributed to sex differences in the effects; however, this route can be used to admin-distribution of subcutaneous fat, since fat is a rel- ister drugs for systemic action. Few drugs readilyatively poorly perfused tissue. penetrate intact skin. The absorption of drugs that do penetrate the skin is proportional to the surface area over which they are applied and toSubcutaneous administration their lipid solubility. Increased cutaneous bloodNon-irritant drugs can be administered by sub- flow also enhances absorption. Systemic toxicitycutaneous (s.c.) injection. The rate of absorption can become evident when highly lipid-solublemay be sufficiently constant and slow to provide substances (e.g. lipid-soluble insecticides) area sustained effect. absorbed through the skin. Controlled-release patches are now commonly used in human med- icine for transcutaneous drug administration.Oral administrationAbsorption from the gastrointestinal tract is gov- Other routes of administrationerned by a number of factors, such as the physicalstate of the drug, the surface area for absorption, Intraarticular administration, local infiltrationblood flow to the absorption site and the drug and ocular administration are useful in equineconcentration at the absorption site. Most drug medicine and are covered in Chapters 7, 15, andabsorption is by passive diffusion; consequently, 13, respectively.absorption from the gastrointestinal tract is The intraarterial, intrathecal and intraperitonealfavored when a drug is in its unionized, more routes are used only rarely in equine medicine.lipophilic form. In general, the absorption of weakacids is optimal from the acidic environment ofthe stomach whereas the relatively alkaline envi- DISTRIBUTIONronment of the small intestine facilitates theabsorption of weak bases. This is, of course, an Once a drug is absorbed, it distributes to manyoversimplification. tissues, including its sites of action. The pattern of drug distribution reflects the physiological and physicochemical properties of the drug. ThePulmonary administration initial phase of distribution often reflects cardiacDrugs may be administered directly into the res- output and regional blood flow. The drug is deliv-piratory tract for activity on, or through, the pul- ered to the heart, liver, kidney, brain and othermonary epithelium and mucous membranes. well-perfused organs during the first few minutesAccess to the systemic circulation is relatively after absorption. The second much slower phaseenhanced and rapid following administration by of drug distribution involves delivery of the drugthis route because the pulmonary surface area is to skeletal muscle, other viscera, skin and fat.large. A drug solution can be administered as an These tissues may require several minutes toaerosol that is inhaled. The advantage of this route several hours before steady-state concentrationsof administration is the almost instantaneous are reached.
  • 9. 1. BASIC PRINCIPLES OF VETERINARY PHARMACOLOGY FOR EQUINE PRACTITIONERS 5 Distribution can be limited by binding of Central nervous systemdrugs to plasma proteins, particularly albumin, Entry of drugs into the cerebrospinal fluid (CSF) andfor acidic drugs and oq-acidglycoprotein for basic extracellular space of the central nervous systemdrugs. Protein-bound drug is too large to passthrough biological membranes; drug that is exten- (CNS) is relatively restricted. The endothelial cells of the CNS have tight junctions and do notsively protein bound will have limited access to have intercellular pores and pinocytotic vesicles.intracellular sites of action (Martinez 1998a).Bound and free drug are in equilibrium on eitherside of membranes and free (unionized) drug is Bonein equilibrium across cell membranes. The ability Divalent metal ion chelating agents (e.g. tetra-of a bound drug to contribute to these equi- cyclines) and heavy metals accumulate in bonelibriums is determined by the strength of its by adsorption onto the bone-crystal surface andadherence to the protein moiety. eventual incorporation into the crystal lattice. Protein-bound drugs may be metabolized andeliminated more slowly than expected. They may Placental transferalso accumulate in tissues in greater concentrationsthan would be expected from their physico- If drugs are transferred across the placenta, theychemical properties because of intracellular bind- may be potentially hazardous to the fetus. Drugsing, partitioning into lipid and ion trapping (pH primarily cross the placenta by passive diffusion.partitioning). This may provide a reservoir that Lipid-soluble, unionized drugs can enter the fetalprolongs drug action. Protein binding also limits bloodstream. The fetal bloodstream is protectedthe glomerular filtration of a drug. However, it from drugs that are relatively lipid insolubledoes not generally limit renal tubular secretion or and/ or highly ionized in plasma. However, thebiotransformation, since these processes lower fetus is, at least to some extent, exposed to essen-the free drug concentration in plasma, which rap- tially all drugs administered to the pregnant mare.idly causes dissociation of the drug-protein com-plex and increases the total unbound fraction. Transcellular compartments The degree of protein binding is only clinicallysignificant for those drugs that are more than 90% Drugs may accumulate in the transcellular fluids.protein bound. For these drugs, conditions that The major transcellular fluid compartment is indecrease plasma protein concentrations, such as the gastrointestinal tract. Weak bases can enterliver and kidney disease, will cause significant the stomach from the circulation and concen-increases in the amount of free drug available trate. Drugs may also be secreted in bile eitherfor pharmacological actions. Protein binding in an active form or as a conjugate that can bemay also be involved in drug interactions. If a hydrolyzed in the intestines. Thus, the gastro-highly protein-bound drug is coadministered intestinal tract can serve as a drug reservoir. Underwith a drug that uses the same protein-binding normal circumstances, the CSF, aqueous humor,site, it can displace the first drug from the plasma and joint fluids generally do not accumulateprotein and thus increase the amount of the significant total quantities of drugs.first drug available for pharmacological action.The classical example of this is the interactionbetween phenylbutazone and warfarin, where BIOTRANSFORMATIONphenylbutazone displaces warfarin from theprotein-binding site. Reducing the amount of Biotransformation detoxifies and/or removesprotein-bound warfarin from 99% to 98% effec- foreign chemicals from the body and thus promotestively doubles the plasma concentrations of free survival. The main metabolic enzyme systemswarfarin (which has anticoagulant activity) and are located in the hepatic smooth endoplasmiccan lead to bleeding problems. reticulum. Some enzyme systems are also located
  • 10. 8 EQUINE CLINICAL PHARMACOLOGYin the kidneys, lungs and gastrointestinal epithe- filtration, active tubular secretion and passivelium. The first-pass effect (presystemic meta- tubular reabsorption. The amount of drug filteredbolism) can thus be a combination of gastrointestinal through the glomeruli is dependent on the degreeand hepatic enzyme systems. Biotransformation of plasma protein binding and the glomerularcan also improve therapeutic activity by activa- filtration rate. Organic anions and cations areting an inactive prodrug or by converting a drug added to the glomerular filtrate by active tubularto a more active metabolite (e.g. metabolic activa- secretion in the proximal tubule. Most organiction of ceftiofur to desfuroylceftiofur). acids (e.g. penicillin) and glucuronide metabolites The chemical peculiarities of drug molecules are transported by the same system that is usedthat allow for their rapid passage across cellular for excretion of natural metabolites, such as uricmembranes during absorption and distribution acid. Organic bases are transported by a separatealso delay elimination. The enzymatic biotransform- system that is designed to excrete bases such asation of drugs into more polar, less lipid-soluble choline and histamine. Both of these systems are(more water-soluble) metabolites promotes elimin- relatively non-selective and may be bidirectional,ation. Conjugation of drugs further increases their with some drugs being both eliminated and reab-water solubility and hence elimination. Horses tend sorbed. However, the main direction of transportto conjugate drugs to glucuronide (Dirikolu et al is into the renal tubules for excretion. Clearly, the2000,Harkins et al2000). rate of passage into the renal tubules is dependent on the pKa of the drug and its metabolites and on urine pH. As an example, increasing urine pH canELIMINATION produce a dramatic increase in excretion of acidic compounds such as salicylate.Drugs are eliminated from the body eitherunchanged or after biotransformation (metabo- GASTROINTESTINAL EXCRETIONlites). The excretory organs tend to eliminate polar,less lipid-soluble compounds more efficiently Many metabolites produced by hepatic meta-than compounds with high lipid solubility. bolism are eliminated into the intestinal tract via The kidney is the most important organ for the bile. These metabolites may be excreted in feceselimination of drugs and metabolites. Some sub- but are often reabsorbed. Organic anions (i.e.stances are excreted via the gastrointestinal tract, glucuronides) and cations are actively transportedfor the most part drugs that were unabsorbed or into bile by carrier systems that are similar to thosemetabolites excreted in the bile and not reab- in the renal tubules. Similarly, charged ions cansorbed. Pulmonary excretion is important mainly compete for transport by these systems becausefor the elimination of anesthetic gases. The both are non-selective. Steroidal and relatedefficacy of some drugs may be partially dependent substances are transported by a third carrieron excretion in the pulmonary secretions. Drugs mechanism. Glucuronide-conjugated metabolitesare also excreted into milk, skin, sweat, saliva undergo extensive enterohepatic recirculation-and tears but these routes are usually of minor a cycle of absorption from the gastrointestinalimportance. In lactating mares, excretion of drugs tract, metabolism in the liver and excretion inin milk may be significant and affect suckling foals. bile-and this cycle delays elimination when theElimination by the latter routes is via diffusion of final step in elimination from the body is via thethe unionized lipid-soluble moiety through the kidneys.gland epithelial cells and is pH dependent. EXCRETION IN MILKURINARY EXCRETION Suckling foals may potentially receive high dosesElimination of drugs and metabolites by the of drug via milk. Normal milk is more acidickidneys is by three mechanisms: glomerular than plasma. Weak bases are highly unionized in
  • 11. 1. BASIC PRINCIPLES OF VETERINARY PHARMACOLOGY FOR EQUINE PRACTITIONERS 7plasma and the unionized drug fraction crosses The total systemic clearance is the sum of clear-readily into the mammary gland and then becomes ance by all mechanisms (e.g. renal, hepatic, etc.)."ion trapped" in the more acidic milk. Weak Clearance is defined as the volume of plasma thatacids are highly ionized in plasma and, therefore, is completely depleted of drug per unit time todo not penetrate the mammary glands very well. account for the rate of elimination. It is usuallyThese factors are used in the systemic treatment constant for a drug, within the desired clinicalof mastitis but should be borne in mind when treat- concentrations, but does not indicate how muching lactating mares with any therapeutic agent. drug is being removed. Clearance = kel / Cp where ke1 is the rate of elimination and Cp is thePHARMACOKINETICS concentration in plasma. Excretion systems have a high ceiling and so are rarely saturated underClinical pharmacokinetics assumes that there is clinical circumstances. The elimination rate isa relationship between the serum concentration essentially a linear function of the drug concentra-versus time profile and the response to a drug. Drug tion in plasma; elimination follows first-orderconcentration versus time data can be described kinetics. This means that there is a constant fractionusing compartmental (model-dependent) or of drug eliminated per unit time. This is whynon-eompartmental (model-independent) pharma- doubling the dose (e.g. of intravenous penicillin)cokinetics. The majority of publications use com- does not reduce the need for frequent dosing. Ifpartmental pharmacokinetics, where the models elimination mechanisms become saturated, theused to describe the drug concentration versus time kinetics follow a zero-order pattern and a constantdata assume that drug in the central compartment quantity of drug is eliminated per unit of time.(blood and rapidly equilibrating tissues) is distrib- In this situation, clearance varies with time.uted to one or more peripheral or tissue compart-ments and that drug is eliminated only fromthe central compartment. A graph of the plasmadrug concentration (on a logarithmic scale) versus HALF-LIFEtime (on a linear scale) following intravenous The elimination or terminal half-life is the timeadministration can be divided into a series of linear needed for the drug concentration in plasma tosegments and can be described using mono-, decrease by 50%. For most drugs, the half-lifebi- or triexponential equations, which reflect the remains constant for the duration of the drugnumber of compartments in the model. One-, two- dose in the body. The elimination half-life can beand three-compartment models have been used calculated following intravenous drug administra-to describe drug disposition in the horse. Some tion whereas the terminal half-life is calculatedauthors use the non-eompartmental approach, following drug administration via other routeswhich is based on statistical moment theory. (non-intravenous). The three most important indices in pharmaco-kinetics are clearance (related to the rate of elimi- Half-life = O.693/kelnation), apparent volume of distribution (Vd : the where 0.693is the natural logarithm of 2. Terminalvolume into which a drug distributes in the half-life is calculated using ~n the terminal slopebody) and bioavailability (the fraction of a drug of the concentration versus time profile for theabsorbed into the systemic circulation). nth compartment, in place of kel Clinically, the half-life determines the inter- dosing interval, how long a pharmacological orCLEARANCE toxic effect will persist and drug withdrawal timesClearance can be used to design an appropriate in food-producing animals and performance horsesdosage regimen for long-term drug administration. (Martinez 1998b). The plasma concentration of
  • 12. 8 EQUINE CLINICAL PHARMACOLOGYdrug remaining at any given time (Cp(t)) can be VOLUMES OF DISTRIBUTIONcalculated as: The Vd is used as an indication of a drugs ability to escape the vascular compartment. It is a mathe- matical term that describe the apparent volume in the body into which a drug is dissolved withwhere CpU = 0) equals the plasma concentration units in milliliters or liters per kilogram (ml/kgof drug at time zero and t is the time interval. or l/kg) (Riviere 1999). The apparent V d valuesTable 1.2 shows the relationship between half-life vary with the physical characteristics of theand the amount of drug in the body. With each drug molecules, including ionization (pKa ), lipidhalf-life, the amount of drug remaining reduces solubility, molecular size and protein binding,by 50%. Note it takes 10 half-lives to eliminate that determine their ability to cross membranes99.9% of a drug from the body. Also recognize (Martinez 1998a).that doubling the dose of a drug (so that the table To understand the concept of Vd , think of thewould start at 200%) does not double the with- body as a beaker filled with fluid (Fig. 1.1), wheredrawal time but merely adds one additional the fluid represents the plasma and the otherhalf-life to reach the same concentration end- extracellular fluid (ECF). If the drug does notpoint (Baggot 1992). Steady-state concentrations readily cross lipid membranes, it will be confinedare reached after three to five half-lives have mainly to the ECF. If this drug is administeredelapsed. intravenously, it will distribute rapidly in the ECF, as represented by the stars in the beaker on theMEAN RESIDENCE TIME left. A sample taken from this beaker will have a high drug concentration. The higher the meas-The mean residence time is the equivalent of half- ured concentration in relation to the originallife and is the parameter calculated when non- dose, the lower the numerical value of Vd . Somecompartmental methods are used to determine drugs readily cross lipid membranes and distri-pharmacokinetic values. Some pharmacokinetic bute into tissues. This is represented by thestudies report mean residence time instead of beaker on the right, where the stars at the bottomhalf-life. The mean residence time is actually the of the beaker represent drug molecules that havetime taken for the plasma drug concentration todecrease by 63.2% and should thus be somewhatgreater than half-life. Table 1.2 Relationship of the elimination half-life to the amount of drug In the body Number of Fraction of drug half-lives remaining (%) o 100 1 50 2 25 3 12.5 Figure 1.1 The volume of distribution of a drug in the body 4 6.25 can be compared with the stars in the beakers. For a drug 5 3.125 that stays confined to the vascular system (the fluid) in the 6 1.56 beaker on the left, taking a sample will yield a high drug 7 0.78 concentration. For a drug that leaves the vascular system and 8 0.39 0.195 binds to tissues, as in the stars bound to the ovals in the 9 10 0.0975 beaker on the right, then sampling the fluid will yield a lower concentration.
  • 13. 1. BASIC PRINCIPLES OF VETERINARY PHARMACOLOGY FOR EQUINE PRACTITIONERS 9been taken up by tissues. A sample taken from quoted in most reference texts and is the valuethe fluid in this beaker will have a low drug con- when the rate of drug entry from the bloodstreamcentration in proportion to the original dose and into the tissues is equal to its exit rate from the tis-will have a high numerical value for Yd sues back into the circulation. It is directly pro- The three most important volume terms used portional to the tissue-binding affinity and thein compartmental analysis are the volume of the fraction of drug remaining unbound in the blood.central compartment (Ve), the volume of distribu- Adult animals are considered to be approxi-tion based on elimination (Vd or Vd~ or Vdarea) mately 60% water; the total body water has aand the volume of distribution at steady state volume of 0.61/kg. The ECF compartment is(VdSS). The numerical values of these terms differ approximately 30% of body weight and has a vol-slightly, but they all give an indication of a drugs ume of 0.31/kg. Neonates are considered to haveability to cross membranes. Although similar in total body water closer to 80% and the additionalmagnitude, Vd is usually greater than Vdss 20% is found primarily in the ECF compartment.because the latter describes the volume that the Geriatric animals tend to have reduced total bodydrug occupies after equilibration has taken place, water primarily because of a reduction in ECFwhich is usually greater than Ve. The value of volume.Vdss and hence clearance and dose rate can also Compounds may be distributed and confinedbe calculated using non-compartmental methods. to the vascular space or may be distribute into The term Ve is the proportionality constant intracellular and extracellular compartments. Inthat relates the drug dose to the amount of drug the latter, Vdss approximates the volume associ-measured in a blood/plasma sample (Cp) ated with total body water. Weakly lipid-soluble For a one-compartment model, the distribution compounds (e.g. cephalosporins, aminoglyco-phase occurs rapidly compared with elimination: sides, penicillins) generally penetrate poorly into cells. Their natural distribution space tends to beVc = Dose/CpU = 0) extracellular and the value of Vdss approximates extracellular fluid volume, about 30% of an ani-In this model, the drug concentration in the blood mals body weight (Baggot 1990). The value foror plasma is proportional to both the concentra- gentamicin approximates 0.2541/kg (Pedersoli ettion in other tissues (e.g. muscle) and to the total al 1980), which implies an ECF distribution,amount of drug in the body. When the equilibra- while that for metronidazole is 0.691/kg (Spechttion of a drug between the central and peripheral et aI1992). Highly lipophilic compounds are asso-compartments occurs less rapidly, relative to ciated with large Vdss values (e.g. propranolol iselimination, then the disposition kinetics of the 3.91/kg). Therefore, a drug with Vd of 0.31/kgdrug can be described by assuming that there are will be distributed primarily into the ECF, whiletwo (or sometimes more) distribution compart- a drug with a Vd of 3.41/kg will be distributedments. The apparent Vd varies with time after beyond the body water compartments and willdrug administration in these models because achieve high concentrations in tissues yet rela-of the time required for equilibration between tively low concentrations in plasma. Some ofthe compartments. Thus, for a two-compartment these drugs (e.g. macrolides, fluoroquinolones)model, Ve is dose/ (A + B),where A and B are the are sequestered within cells or have extensive tis-y intercepts (plasma concentrations at time 0) sue binding, reflected by estimates of Vdss thatassociated with the distribution and elimination exceed the volume of total body water (Atkinsonphases, respectively. et aI1996). If the value of Vd is greater than l1/kg,Vd = Dose/(I3·AUC) the drug concentration will be greater in tissues than in plasma (Evans 1992). While a large valuewhere ~ is the slope of the terminal phase and AVC for Vdss suggests excellent extravascular distri-is the area under the plasma concentration versus bution, it does not guarantee adequate active drugtime curve from time 0 to infinity. The Vdss is concentrations at the sites of action.
  • 14. 10 EQUINE CLINICAL PHARMACOLOGY Drug doses are usually determined in normal,healthy adult animals and the Vd value is constant OTHER CLINICALLY RELEVANTfor any drug and only changes if there are CONCEPTSphysiological or pathological changes that alterdrug distribution. Any condition that changes THE LOADING DOSEECF volume will dramatically affect the plasma A loading dose is an initial dose of drug that isconcentrations of drugs with low Vd values. Drugs administered at a dose rate higher than that nor-with high Vd values normally distribute through- mally used. A loading dose is used when the timeout the fluid and tissue compartments and so are needed to reach steady-state concentrations (i.e.not significantly affected by changes in body water about three to five half-lives) is long relative to thestatus. ECF volume contraction and dehydration need for treatment. It rapidly increases plasmaoccur in conditions such as shock, colic and enteri- concentrations so that steady-state concentrationstis. Parasitism, heart failure and vasculitis can all are approached more rapidly.cause edema and an increase in the ECF volume.Local changes in acid-base status can alter the ion-ization of drugs and affect their movement across MAXIMUM PLASMAmembrane barriers. Conditions that alter the CONCENTRATIONamount or affinity of plasma proteins will change The maximum plasma concentration reflects thethe V d value of highly protein-bound drugs. extent of drug bioavailability. It can be used in rela- tion to minimum inhibitory concentration (MIC) to predict the efficacy of concentration-dependentBIOAVAILABILITY antimicrobial agents (e.g. fluoroquinolones, amino- glycosides). Both the maximum (peak) and mini-Bioavailability is a measure of the extent of absorp- mum (trough) plasma concentrations are usedtion. It is an indication of the amount of drug that during therapeutic drug monitoring to maximizeis absorbed from the administration site. Drugs efficacy and minimize the occurrence of undesir-administered intravenously essentially have a able effects.bioavailability of 100%. Drugs administered orallywill have variable bioavailability. The rate ofabsorption is clearly important because if the rate REFERENCESis very slow, the drug may not reach active con- Atkinson A J, Ruo T, Frederiksen M C 1996 Physiologicalcentrations before it is eliminated and if it is very basis of multicompartmental models of drug distribution.rapid, unsafe plasma concentrations may be Trends in Pharmacological Sciences 12:96-101reached. Baggot J D 1990 Pharmacokinetic-pharmcodynamic relationship. Annals de Recherches Veterinaires AVC is a frequency distribution of the number 21(suppl):29-40of molecules within the body versus time. When Baggot J D 1992 Bioavailability and bioequivalence ofmeasured out to infinity (00), the AVC value veterinary drug dosage forms, with particular reference to horses: an overview. Journal of Veterinary(AVC()...oo) represents the total drug exposure. Its Pharmacology and Therapeutics 15:160-173value is unaffected by the rate of absorption Dirikolu L. Lehner A F, Karpiesiuk W et al 2000 Identification(assuming linear kinetics) but is affected by dose, of lidocaine and its metabolites in post-administration equine urine by ELISA and MS/MS. Journal of Veterinaryclearance and bioavailability. Bioavailability is Pharmacology and Therapeutics 23:215-222calculated by comparing the total amount of drug Evans W E 1992 General principles of appliedin the body (AVC) following administration by pharmacokinetics. In: Evans W E, Schentag J J, Jusko W J et al (eds) Applied pharmacokinetics, principles ofa non-i.v, route with that obtained following i.v, therapeutic drug monitoring, 3rd edn. Appliedadministration (100%), corrected for dose. Therapeutics Inc, Vancouver, pp. 1-8 Harkins J D, Karpiesiuk W, Tobin T et al 2000 Identification AUCnon-Lv, x Dose.; of hydroxyropivacaine glucuronide in equine urine byBioavailability = - - - - - - - ESI+/MS/MS. Canadian Journal of Veterinary Research AUCi,v, X Dosenon-i,v, 64:178-183
  • 15. 1. BASIC PRINCIPLES OF VETERINARY PHARMACOLOGY FOR EQUINE PRACTITIONERS 11Martinez M N 1998a Physicochemical properties of concentration in body fluids and endometrial tissues of pharmaceuticals. Journal of the American Veterinary mares. American Journal of Veterinary Research Medical Association 213:1274-1277 53:1807-1812Martinez M N 1998b Volume, clearance and half-life. Journal of the American Veterinary Medical Association 213:1122-1127Pedersoli W M, Belmonte A. Purohit R C et al1980 SUGGESTED READING Pharmacokinetics of gentamicin in the horse. American Journal of Veterinary Research 41:351-354 Gilman A G, Rail T W, Nies A S et al (eds) 1990 TheRiviere J E 1999 Comparative pharmacokinetics: principles, pharmacological basis of therapeutics, 8th edn. techniques and applications. Iowa State University Pergamon Press, New York Press, Ames, lA, pp. 47-61 Rowland P E, Towser T N (eds) 1995 ClinicalSpecht T E, Brown M P, Gronwall R R et al 1992 pharmacokinetics: concepts and applications. Lea and Pharmacokinetics of metronidazole and its Febiger, Philadelphia, PA
  • 16. CHAPTERCONTENTSRational use of antimicrobial agents 13Documenting an infection 17 Antimicrobial therapyDosage regimen design 18 Inhibitory drug dosage 18 Patricia M. Dowling Bacteriostatic versus bactericidal agents 19 Antimicrobial drug concentration 20 Calculating the drug dosage regimen 20 Site of the infection 21 Combination antimicrobial therapy 21 Prophylactic use of antimicrobial drugs 22Beta-Iactam antibiotics 22 Penicillin G (benzylpenicillin) 23 Aminopenicillins: amoxicillin and ampicillin 24 Extended-spectrum penicillins 25 Cephalosporins 25 Imipenem 28 RATIONAL USE OFAminoglycoside antibiotics 28 ANTIMICROBIAL AGENTS Streptomycin/dihydrostreptomycin 32 Neomycin 32 In recent years, there have been important changes Gentamicin 32 Amikacin 33 in antimicrobial therapy. New antimicrobial agents Tobramycin 33 have become available and there is a greater database of species-specific pharmacokinetic andChloramphenicol and florfenicol 33 pharmacodynamic information on the agentsPotentiated sulfonamides 35 used in veterinary medicine. Concerns over drug residues in food-producing and performance ani-Tetracyclines 38 mals and the continued development of bacterialFluoroquinolones 40 resistance to antimicrobial agents have heightened the awareness of the rational use of these agents.Macrolides 43 By definition, antibiotics are natural products ofRifampin 44 microorganisms. The term antimicrobial agent is more encompassing as it includes synthetic drugs,Metronidazole 45 such as the fluoroquinolones, and organic com-Vancomycin 45 pounds, such as the sulfonamides. The equine practitioner must consider severalReferences 46 questions when developing an antimicrobial treat- ment regimen • does the diagnosis warrant antimicrobial therapy? • which organisms are likely to be involved? • what is the in-vitro antimicrobial susceptibility of the pathogen? • where is the infection located and is it accessible to the drug? • will the agent be effective in the environment at the site? • what formulation and treatment regimen will maintain the appropriate antimicrobial concentrations for sufficient time? 13
  • 17. 14 EQUINE CLINICAL PHARMACOLOGY• what adverse reactions or toxicities might walled off by a thick fibrous capsule. It is difficult occur? for many drugs to reach therapeutic concentra-• is there a product suitable for horses? tions in the central nervous system (CNS), mam- mary glands and accessory sex glands because ofDoes the diagnosis warrant specialized blood and tissue barriers.antimicrobial therapy? Chapter 1 discussed the principles of drug pharmacokinetics; here the importance of volumesMuch of the use of antimicrobial agents is irra- of distribution (Vd), drug ionization and proteintional. Antimicrobial therapy for superficial binding are outlined for antimicrobial drugs.wounds and single doses of penicillin adminis-tered after the elective castration of normal horses Volumes of distributionhas no real therapeutic benefit and encouragesantimicrobial resistance. The Vd is a pharmacokinetic parameter that indi- cates the apparent distribution of a drug withinWhich organisms are likely to be the body (Riviere 1999). The relative value of Vdinvolved? indicates how well a drug is going to distribute to the tissues. Antimicrobial agents can be catego-For many equine infections, it is possible to predict rized as having Vd values that are low «0.31/kg,which microorganisms will be involved. Traumatic similar to the volume of the extracellular fluidwounds are usually contaminated with skin and (ECF)), medium (0.3-11/kg) and high (>ll/kg,fecal flora. Respiratory tract infections routinely greater than the total body water volume) (Tableinvolve Streptococcus (Strep.) species (spp.). Septic 2.1 and the Appendix to this chapter).neonates routinely have positive blood cultures Physiological or pathological changes in thefor Escherichia coli and other Gram-negative enteric Vd are very important in determining the dose ratebacteria. of drugs that are predominantly confined to the ECF compartment. Foals have a total body waterWhat is the in-vitro antimicrobial of 80%, whereas adult horses are 60% water. Forsusceptibility of the pathogen? a given dose of any drug with a low Vd , such as gentamicin, foals will have a lower plasma con-The antimicrobial susceptibility of many patho- centration than adult horses. Therefore, a highergens is very predictable (Prescott & Holgate 1993). dose must be given to foals to achieve the sameFor example, Streptococcus spp. and most anaer- effective plasma drug concentrations (Fig. 2.1).obes are usually susceptible to penicillin. However, This is not intuitive; it is commonly thought thatpathogens such as Gram-negative enteric bacte- neonates should be given a lower dose than adultria are very efficient at transmitting and acquiring horses because of concerns about gentamicinresistance genes and, therefore, have unpredictablesusceptibility patterns: susceptibility testing shouldalways be carried out in order to choose effective Table 2.1 Categories of antimicrobial agents basedtherapy. on their volume of distribution (Veil Volume of distribution (I/kg)Where is the infection locatedand will the agent reach the site of Low «0.3) Medium (0.3-1) High (>1)infection? Penicillins Sulfonamides Fluoroquinolones Cephalosporins Florfenicol TrimethoprimPractitioners must consider the effects of physiol- Aminoglycosides Tetracyclinesogy and pathology on the distribution of drugs Macrolidesin order to adjust dosages to achieve effective Chloramphenicol Metronidazoletherapy. Many infections occur in sequestered Rifampinareas of the body. For example, an abscess may be
  • 18. 2. ANTIMICROBIAL THERAPY 16nephrotoxicity. This can unfortunately lead to Clinically, it is sufficient to remember that onlyunder-dosing and ineffective therapy. nonionized drug crosses membranes readily and that "like is nonionized in like". This concept dic- tates the distribution of antimicrobial agents intoDrug ionization sequestered infections. For example, mastitis isChanges in acid-base balance are very common treated parenterally with antimicrobial agents thatin disease states. Antimicrobial drugs are weak are weak bases (Fig. 2.2) because milk is moreacids or weak bases (Table 2.2, Appendix). Their acidic than plasma. Weak bases are highly union-lipid solubility depends greatly on their degree ized (UI) in plasma and thus can cross readily intoof ionization (charged state) (Martinez 1998a).An the mammary gland, where they become "ionionized drug is hydrophilic and poorly lipid sol- trapped" in the more acidic environment of theuble. A drug that is nonionized is lipophilic and mammary gland (milk). A new equilibrium iscan cross biological membranes. The degree of established between the ionized and unionizedionization of a weak acid or base depends on drug. Although smaller than the fraction inboth the acidic dissociation constant (pKa ) of the plasma, the unionized fraction in the milk candrug and the pH of the surrounding fluid. enter pathogenic bacteria and produce the desired antimicrobial action. By comparison, weak acids are highly ionized in plasma and, therefore, do not penetrate into the mammary gland very well. Successful therapy of mastitis using agents that are weak acids requires local (intramammary) infu- sion, as is routinely done in cattle, where the high local drug concentrations overcome any effects of drug ionization. Other sequestered infections such as abscesses, metritis and meningitis are also typi- cally acidic compared with plasma; consequently,Figure 2.1 For a drug with a low volume of distribution,when the same dose is given to an adult horse and a foal, theincreased extracellular fluid volume of the foal dilutes the drug.resulting in lower plasma concentrations. To achieve the sameplasma concentrations. the foal must be given a higher dosethan the adult horse. Table 2.2 Classification of antimicrobial agents according to their dissociation constant (pIC.) Acidic drugs Basic drugs Amphoteric drugs Mammary gland Figure 2.2 A weak base is not highly ionized in plasma and this Penicillins Aminoglycosides Fluoroquinolones fraction is availableto cross the blood--mammary gland barrier Cephalosporins Macrolides Tetracyclines Sulfonamides Chloramphenicol/ and enter the milk. A new equilibrium is established between florfenicol unionized (UI) and ionized (I) drug in the more acidic milk. The Trimethoprim unionized fraction in milk is available to cross the cell wall of pathogenic bacteria to produce desired antimicrobial action.
  • 19. 16 EQUINE CLINICAL PHARMACOLOGYparenteral therapy is most effective using basic where a new equilibrium is established betweenantimicrobial agents. the free and bound drug. Because of its high degree Acidic antimicrobial agents are also drugs with of protein binding, ceftiofur does not readilylow Vd values and most weak bases have high Vd cross into milk when administered parenterally,values (Table 2.2). The exception to this rule is the hence the "zero" milk withdrawal time in lactat-aminoglycosides. The aminoglycosides are weak ing dairy cattle. There is no interaction with otherbases but are very large, hydrophilic molecules highly protein-bound drugs, such as phenylbuta-that are highly ionized at physiological pH and zone, since ceftiofur and phenylbutazone do notdo not readily cross lipid membranes. Therefore, compete for the same protein-binding site.aminoglycosides that are administered parenter-ally do not achieve therapeutic concentrations Will the antimicrobial be effectivein milk, abscesses or cerebrospinal fluid (CSF). in the infection site environment?Amphoteric drugs, such as the fluoroquinolonesand tetracyclines, have both acidic and basic Even when the pharmacokinetic parameters of agroups as part of their chemical structures. These drug are such to suggest that it will reach the sitedrugs have a range of pH where they are maxi- of the infection, local factors can influence itsmally VI. For example, fluoroquinolones are most antimicrobial activity. Aminoglycosides are inef-lipid soluble in the pH range 6-8 and so are fective in hyperosmolar, anaerobic acidic envi-lipid soluble in most situations. These drugs are ronments, such as the purulent environment ofsignificantly ionized in acidic urine, which reduces abscesses. Sulfonamides act by replacing para-their antibacterial activity. However, the fluoro- aminobenzoic acid (PABA) in the folic acid syn-quinolones are primarily eliminated in urine, so thetic pathway of bacteria and are ineffective inany reduction in activity caused by low environ- purulent material and necrotic tissue, which pro-mental pH is offset by the extremely high urine vide alternative sources of PABA.concentrations of these agents and is thus of noclinical importance. The fluoroquinolones and What drug formulation andtetracyclines are highly VI at most physiological treatment regimen will maintainpH, so are similar to weak bases in that they have appropriate antimicrobialhigh Vd values. concentrations for sufficient time? The formulation of a drug influences its availabil-Protein binding ity to the systemic circulation (Baggot 1992).Many drugs are bound to plasma proteins (mainly Administration intravenously (i.v.) achieves thealbumin) in the circulation, bound drug is too most rapid onset of drug action. With intramuscu-large to pass through biological membranes, so lar (i.m.) or subcutaneous (s.c.) injections, absorp-only a free drug is available for delivery to the tis- tion can be delayed as drug moves from thesues and to produce the desired pharmacological injection site into the vascular system. The absorp-actions (Martinez 1998a). However, just like the tion rate can also vary depending on the site ofrelationship between ionized and VI drugs, free injection (e.g. absorption is usually more rapidand bound drugs are in equilibrium. from the neck muscles than from the muscles of the The degree of protein binding is only clinically hindquarters). Some formulations are designed tosignificant for those drugs that are more than have a slow release profile from the injection site90% protein bound. The only antimicrobial agent making the antimicrobial agent "long acting".that is significantly protein bound is ceftiofur. Preparations for oral (p.o.) administration mayThe efficacy of ceftiofur is attributed to its binding have reduced or erratic systemic availability into acute-phase proteins, such as Cil-antitrypsin, herbivores because of adsorption onto feedstuffswhich acts as a reservoir of the active drug and and incomplete absorption. The liver may inacti-carries the bound drug to sites of inflammation, vate some drugs that are easily absorbed as they
  • 20. 2. ANTIMICROBIAL THERAPY 17pass via the portal circulation (first-pass effect). Is there a suitable productDrugs with large molecular weights may not be approved for use in horses?well absorbed, unless attached to a "carrier" that In addition to finding a suitable product, the prac-allows absorption through the lymphatic system. titioner must also determine an appropriate withdrawal time for an antimicrobial agent usedElimination half-life in a horse intended for human consumption or performance.Infectious diseases are typically treated using Whenever possible, veterinarians should usemultiple doses of an antimicrobial agent. The tim- approved products. If there is no suitableing of these repeated doses is determined by theelimination half-life of the drug (Martinez 1998b). approved product, then agents approved for use in other species may be used in horses as long as thereThe elimination half-life is the time required for is a valid veterinarian-elient-animal relationship.the drug concentration to decrease by one-half. The veterinarian must be available in the case ofFor most drugs, half-life remains constant for the treatment failure or adverse reactions and mustduration of the drug dose in the body. Clinically,the half-life determines (a) the inter-dosing inter- be able to provide information on the withdrawalval; (b) how long a pharmacological or toxic effect time prior to slaughter or competition. For informa- tion on withdrawal periods, veterinarians shouldwill persist; and (c) drug withdrawal times forfood-producing animals and performance horses. consult a database such as the Food Animal Residue Avoidance Databank (FARAD) in the USA (Riviere et al 1998). FARAD centers are beingWhat adverse drug reactions or established worldwide. Drug withdrawal timestoxicity might be expected? for performance horses will vary between sportsMost drugs have some potential adverse/toxic and countries. The governing body of a particu-effects and the practitioner must decide if the risks lar discipline should be consulted for the appro-of therapy outweigh the benefits. Antimicrobial priate current guidelines.drugs frequently cause adverse reactions in horses.Because of their digestive physiology, horses areprone to developing antimicrobial-associated coli-tis that can be fatal (Freeman 1999).Antimicrobial DOCUMENTING AN INFECTIONagents with activity against anaerobic bacteria,combinations of broad-spectrum agents and those A diagnosis must be established before any ther-agents that undergo extensive enterohepatic recir- apy can be administered. It is not always neces-culation are often incriminated in disturbing the sary to isolate bacteria from all horses withnormal gastrointestinal flora and allowing the microbial infections in order to identify the organ-proliferation of Clostridium spp. and pathogens isms involved. The diagnosis can be based onsuch as Salmonella spp. Anaphylactic reactions and clinical experience. The signs of some particularimmune-mediated hemolytic anemia are associ- infections are so obvious that the need for micro-ated with penicillin administration. Fatal cardiac biological confirmation is minimal; however, forarrhythmias are associated with the concurrent those diseases of unknown cause or attributablei.v, administration of potentiated sulfonamides to organisms with unpredictable antimicrobialand the ClTadrenergic agonist detomidine. susceptibility, there is no substitute for bacterial The use of antimicrobial agents for relatively culture and identification of the causative agent.trivial infections encourages the selection of For these organisms, initial therapy, while waitingresistant organisms. Consequently, in the absence for the culture results, should include an anti-of evidence of a susceptible pathogen, antimicro- microbial agent with a broad spectrum of activity,bial use is irrational and may expose treated although these agents are usually more toxic andhorses to unnecessary risks. more expensive.
  • 21. 18 EQUINE CLINICAL PHARMACOLOGY Representative samples of infected material MIC values are used to determine a drug dose, thatshould be taken from clinical cases. Beware of should achieve blood and tissue concentrationssampling grossly contaminated sites, such as that exceed the in vitro MIC of the pathogen.wounds and purulent nasal discharges. Correct Antimicrobial susceptibility is described as thesamples may improve the odds of isolating the following:relevant pathogen: septic arthritis is best diag- • susceptible ("5") pathogen is whennosed using synovial fluid samples rather than a MIC < local drug concentration: successfulsynovial membrane biopsy, salmonellosis from therapyrectal biopsy and septicemias from blood culture. • intermediate ("I") pathogen is whenA Gram stain can be performed immediately on a MIC = local drug concentration: doubtfuldirect smear and will direct initial therapy until the therapeutic effectlaboratory results are obtained. Samples should • resistant ("R") pathogen is when MIC > localbe submitted for appropriate culture and identifi- drug concentration: drug will have no effect.cation and the type of culture required should bespecified: aerobic, anaerobic, mycoplasma. In some The "5", "I", "R" designations are assigned bycases, the pathogen may be identified using serol- laboratories based on safely achievable plasmaogy to demonstrate antibodies (e.g. leptospirosis, concentrations. A culture and susceptibility reportbrucellosis, ehrlichiosis). should contain the name of the organism and a list of common antimicrobial agents designated as susceptible, intermediate or resistant. This infor-DOSAGE REGIMEN DESIGN mation is intended to guide antimicrobial selec- tion; however, these reports must be interpretedSuccessful antimicrobial therapy relies on admin- carefully. There are a number of important factorsistering sufficient doses to suppress or kill that are not taken into account in these datapathogens effectively at the site of infection so that (Verbist 1993).they can be eliminated by the hosts defenses.The relationship between the host, the organism Host defensesand the drug can be very complex. High plasmaantimicrobial concentrations are assumed to be The interaction between an antimicrobial and aadvantageous in that a large amount of drug will pathogen in the laboratory does not take normaldiffuse into various tissues and body fluids. The host defense systems into account. Humoral anddrug concentration at the infection site is assumed cell-mediated immune systems playa major roleto be of major importance in determining drug in pathogen eradication; their contribution isefficacy. Remember, the ability of a drug to diffuse underestimated in susceptibility reports. Anti-out of the plasma into the extravascular tissues microbial agents act in concert with endogenousdepends on its molecular size, lipid solubility, pKa, microbial inhibitors such as immunoglobulins,and the degree of protein binding, as well as local T lymphocytes, phagocytes, complement compo-pH and specific cellular transport mechanisms. nents, lactoferrin, lactoperoxidase and lysozymes.INHIBITORY DRUG DOSAGE Drug distribution in the bodyIn the laboratory, the relationship between an Susceptibility designations are based on achievableantimicrobial drug and a pathogen is described plasma concentrations and do not take preferentialby the minimum inhibitory concentration (MIC) drug accumulation at specific sites into account:and the minimum bactericidalconcentration (MBC). most antimicrobial agents are eliminated by theThe MIC is the lowest drug concentration that kidneys, achieving concentrations in urine that areinhibits bacterial growth. The MBC is the lowest hundreds of times higher than those in plasma;drug concentration that kills 99.9% of the bacteria. tetracyclines accumulate in pneumonic lung tissue,
  • 22. 2. ANTIMICROBIAL THERAPY 19resulting in successful therapy, which would not be Table 2.3 Classification of antimicrobial agents aspredicted by in vitro susceptibility testing. Direct bactericidal or bacteriostaticapplication of an antimicrobial drug, such as atopical or an ophthalmic formulation, produces Bactericidal Bacteriostaticsuch high concentrations that the susceptibility Fluoroquinolones Chloramphenicolreport may be inapplicable. Aminoglycosides Florfenicol Penicillins Macrolides Cephalosporins TetracyclinesGrowth rates and the size of Trimethoprim!sulfonamides Sulfonamidesinoculum at the infection site in susceptibility profiles, yet they have great valueMIC is measured using standardized methods and in veterinary medicine. In addition, agents nota standardized inoculum size. In clinical cases, listed in susceptibility reports are often not con-there may be sites of infection with only a few sidered for therapy, even when they may be bothbacteria and other sites with many. Some bacteria suitable and effective.grow and multiply very slowly at infection sites, The use of in vitro MIC values to predict thewhile the laboratory incubator and optimal con- results of antimicrobial therapy in vivo is ques-ditions encourage rapid growth and multiplica- tionable. Yet, by convention, drug dosage regi-tion. Rapidly multiplying bacteria under optimal mens target a plasma drug concentration that istest conditions are very sensitive to antimicrobial based on some multiple (usually 2-10) of thetreatment. MIC. Proposed treatment regimens are then eval- uated in clinical cases and modified, as required,Mixed infections to maximize efficacy.In the laboratory, cultured organisms are separatedprior to susceptibility testing. This prevents the BACTERIOSTATIC VERSUSobservation of any synergism between pathogens. BACTERICIDAL AGENTSFor example, Pasteurella spp. and anaerobes It is common to classify antimicrobial agents asdemonstrate synergistic pathogenicity in vivo that either bactericidal or bacteriostatic (Table 2.3). Ifcannot be seen when they are cultured separately the ratio of the MBC to the MIC is small «4-6),in vitro. the agent is considered to be bactericidal and it is possible to obtain drug concentrations that willInfection environment kill 99.9% of the organisms exposed. If the ratio ofLaboratory culture plates provide an ideal envi- the MBC to the MIC is large, it may not be possi-ronment for the drug-organism interactions. In ble to administer safely dosages of the drug toclinical cases, the local environment at the infec- kill 99.9% of the bacteria and the agent is consid-tion site has a large effect on antimicrobial action. ered to be bacteriostatic. For many drugs, the dis-In diseases characterized by abscess formation, tinction between bactericidal and bacteriostatic istreatment failure may occur when the chosen not exact and depends on the pathogen involvedantimicrobial agent is ineffective in acidic, anaer- and the drug concentration attained in the targetobic and hyperosmolar environments. The action tissues. For example, florfenicol is consideredof many antimicrobial drugs is decreased in milk. bactericidal in very susceptible pathogens of the bovine respiratory tract but bacteriostatic in enteric pathogens. A bactericidal drug may be preferredRoute of administration over a bacteriostatic drug in specific situationsUnless specifically requested, topically adminis- including immunosuppressed patients (e.g. septictered antimicrobial agents are not tested. System- neonates), life-threatening conditions (e.g. bacte-ically toxic antimicrobial agents, such as polymixin rial endocarditis and meningitis) and for surgicalB,bacitracin and mupirocin, are often not included prophylaxis.
  • 23. 20 EQUINE CLINICAL PHARMACOLOGY Table 2.4 Postantlbiotlc effect (PAE) of various antimicrobial agents Postantibiotic effect (h) Long (>3) Intermediate (1-3) Short «1) Gram-positives Fluoroquinolones Aminoglycosides Macrolides Penicillins Chloramphenicol Cephalosporins Tetracyclines Gram-negatives Fluoroquinolones Penicillins Aminoglycosides Cephalosporins Trimethoprim/sulphonamides Anaerobes Metronidazole For some bacteria-drug interactions, bacterial Table 2.5 Concentration-dependent versusgrowth remains suppressed for a period after the time-dependent antimicrobial agentsdrug concentration has decreased below the MIC. Concentration dependent Time dependentThis period when antimicrobial concentrationshave fallen below MIC but the damaged bacteria Aminoglycosides Penicillinsare more susceptible to host defenses is called the Fluoroquinolones Cephalosporinspostantibiotic effect (PAE).PAE may explain why (in Gram-negative aerobes)dosage regimens that allow antimicrobial con- Metronidazole Other "bacteriostatic" antimicrobial agentscentrations to fall below the MIC for a significantportion of the interdosing interval are still effica-cious. The PAE depends on both the antimicro-bial agent and the specific bacterial pathogen CALCULATING THE DRUG(Table 2.4). DOSAGE REGIMEN Antimicrobial dosage regimens are designed in one of two ways: either to maximize the plasmaANTIMICROBIAL DRUG concentration or to provide a plasma concentra-CONCENTRATION tion above the MIC for most of the interdosingBacterial killing curve studies show that antimicro- interval (Vogelman et al 1988).bial agents exhibit either concentration-dependent For concentration-dependent killers with a pro-or time-dependent bacterial killing (Table 2.5) longed PAE, it is suggested that the peak plasma(Martinez 1998c). For concentration-dependent drug concentration be 8- to lO-fold higher thankillers, such as the aminoglycosides, a high max- the MIC of the pathogen. If the V d of the anti-imum plasma concentration (Cmax) relative to the microbial is known, a precise drug dosage regimenMIC is the major determinant of clinical efficacy. for the pathogen can be calculated using the fol-These drugs also have prolonged PAE values, lowing equation:thereby allowing long interdosing intervals that Dose rate = Vd [desired plasma concentration]maximize clinical efficacy and minimize side-effects. For time-dependent agents, such as the For example, a foal with Klebsiella spp. pneumoniapenicillins and cephalosporins, the time that is to be treated with gentamicin. The MIC of gen-the antimicrobial concentration exceeds the MIC tamicin for Klebsiella spp. is 2 f,Lg/ml. The desireddetermines the clinical efficacy. Once the MIC of plasma concentration would be 10 times the MIC,the bacteria has been exceeded, further increases which is 20 f,Lg/ml. The Vd of gentamicin in thein the plasma concentrations do not increase the foal is O.31/kg. The dose rate calculated is:bactericidal activity of these agents. Dose rate = (300ml/kg)(0.02 mg/ml) = 6 mg/kg
  • 24. 2. ANTIMICROBIAL THERAPY 21Given once daily, this dosage would provide effec- COMBINATION ANTIMICROBIALtive concentration-dependent bacterial killing, THERAPYwhile limiting the renal accumulation of gentam- Combination antimicrobial therapy is common-icin and any associated nephrotoxicity. place in equine practice. However, combination For time-dependent killers, the objective is to therapy has never been demonstrated to be supe-keep the average plasma drug concentration above rior to single drug therapy in controlled clinicalthe MIC of the pathogen for the entire duration of trials. The use of multiple antimicrobial agentsthe interdosing interval. Again, utilizing Vd and should be limited to certain situations.half-life, you can calculate a dosing regimen. 1. Combinations with known synergismTherapeutic index against specific organisms. Synergism occursThe therapeutic index assesses a drug in terms of when the antimicrobial effect of a combinationthe ratio between the average minimum effective of drugs is greater than the sum of theirdose and the average maximum tolerated dose in independent effects. For example, penicillinsnormal subjects. This does not take into account and cephalosporins are synergistic withvariation between individuals and it is now usu- aminoglycosides. Disruption of the bacterial cellally defined as the median lethal dose (LDso) wall by the beta-lactam antibiotic allows fordivided by the median effective dose (EDso). It is greater uptake of the aminoglycoside andintended to indicate the margin of safety of a drug, results in a synergistic killing effect.a high therapeutic index implying a better safety 2. To prevent the rapid development ofmargin. bacterial resistance. Erythromycin and rifampin are used in combination in the treatment of foals with Rhodococcus (R.) equi infections. Each drugSITE OF THE INFECTION has a completely different mechanism ofThe pathophysiology of an infection influences the antimicrobial action; their combination reducesdistribution and activity of antimicrobial agents. the chance of chromosomal mutationsAbscess formation poses a significant therapeutic conferring bacterial resistance.problem: the walls of an abscess limit the pene- 3. To extend the spectrum of activity for thetration of non-lipid-soluble drugs; the acidic envi- initial antimicrobial therapy of life-threateningronment encourages weak bases to accumulate; infections. In emergency situations, such asthe low pH and the presence of cellular debris septicemia or meningitis, where the causativeinterfere with the activity of some antimicrobial organism is unknown, combination therapyagents. Penicillins and cephalosporins do not may be initiated to provide antimicrobialpenetrate abscesses well. Aminoglycosides do activity against Gram-positive, Gram-negativenot penetrate abscesses well and are inactivated and anaerobic bacteria.in acidic, anaerobic, hyperosmolar environments. 4. To treat mixed bacterial infections. ForPotentiated sulfonamides achieve adequate con- example, it is rational to combine gentamicincentrations, but the competitive mechanism of plus a beta-lactam plus metronidazole for casesaction of the sulfonamides is overwhelmed by the of equine pleuropneumonia, which predictablyabundance of free PABA from lysed phagocytes. involve mixed infections of Strep. zooepidemicus,The fluoroquinolones and macrolides achieve Gram-negative enteric bacteria and anaerobes.high concentrations, but the acidic environment 5. Non-synergistic or antagonisticlessens their activity. Rifampin (rifampicin), chlo- combinations should be avoided. Classically,ramphenicol, florfenicol, the tetracyclines and penicillins are not administered concurrentlymetronidazole all achieve high concentrations in with tetracyclines. The penicillins requireabscesses and retain their antimicrobial efficacy actively dividing bacterial cells to be effective asin purulent environments. they act against bacterial cell wall formation,
  • 25. 22 EQUINE CLINICAL PHARMACOLOGYwhile the tetracyclines bacteriostatic action surgical patients are immunocompromised toinhibits bacterial replication. The combination some degree.of procaine benzylpenicillin (procaine penicillin) 7. The selected protocol should be cost effective.and a potentiated sulfonamide has minimallyadditive effects against pathogens but doeshave additive effects against the commensalmicroflora; so this combination therapy BETA-LACTAM ANTIBIOTICSincreases the risk of colitis. The beta-lactam antibiotics are commonly usedPROPHYLACTIC USE OF because of their safety, efficacy, relatively lowANTIMICROBIAL DRUGS cost and the variety of dosage forms available. Both penicillins and cephalosporins have a four-The principles upon which drugs are used pro- membered beta-lactam ring that is responsible forphylactically to prevent surgical infections are the instability of these compounds. The penicillinsbased on human studies; there are few veterinary are also relatively insoluble; they are prepared asstudies that evaluate these recommendations. various salts by the substitution of the hydroxyl group or the hydrogen of the carboxyl group1. The relative risk of infection must warrant with sodium, potassium, benzathine or procaine.the use of prophylactic antimicrobial agents.Typically, they are used in surgical proceduresassociated with an infection rate that exceeds Mechanism of action5%. The risks of the prophylactic antimicrobial Beta-lactam antibiotics act on enzymes calledtherapy must be less than the risk and penicillin-binding proteins (PBP)near the bacterialconsequences of infection. cell wall. When beta-lactam antibiotics bind cova-2. The organisms that are likely to cause the lently and irreversibly to the PBP, they interfereinfection and their antimicrobial with the production of cell-wall peptidoglycans,susceptibility should be known or predicted causing cell lysis in hypoosmotic environments.accurately. Routine monitoring in surgical Differences in the spectrum of activity and actionshospitals provides information on the normal of beta-lactam antibiotics result from their relativeequine flora and nosocomial pathogens that are affinity for different PBP.involved commonly in hospital-acquiredinfections.3. The drug must be administered and Resistance mechanismsdistribute to the potential infection site beforethe onset of the infection. To achieve high Resistance to the beta-lactams develops via aconcentrations rapidly, antimicrobial agents are number of mechanisms.administered i.v, to provide prophylaxis.4. Drugs used prophylactically should not be 1. Failure of the antibiotic to penetrate thethose that are used routinely for therapy, to outer layers of bacterial cells. In order to bind toavoid bacterial resistance caused by previous the PBP, the beta-lactam antibiotic must firstexposure to that antimicrobial agent. diffuse through the bacterial cell wall.5. The duration of antimicrobial prophylaxis Gram-negative organisms have an additionalshould be as short as possible. Most studies lipopolysaccharide layer that decreases antibioticdemonstrate that prolonging treatment for penetration. Therefore, Gram-positive bacterialonger than 24 hours after the procedure has no are usually more susceptible to the action ofadditional benefits. beta-lactams than Gram-negative bacteria.6. The selected agent should be bactericidal 2. Genetic alteration of PBP decreases therather than bacteriostatic. It is assumed that affinity of the PBP for the antibiotic. This is the
  • 26. 2. ANTIMICROBIAL THERAPY 23mechanism of resistance in methicillin-resistant from i.rn. and s.c. injection sites. Procaine ben-staphylococci (MRSA). zylpenicillin (procaine penicillin) is more slowly3. Production of beta-lactamase enzymes. There absorbed from i.m. sites than the sodium ormay be as many as 50 beta-lactamase enzymes potassium salts, so it produces lower but more(penicillinases, cephalosporinases) produced by sustained plasma concentrations. Procaine ben-bacteria. These enzymes hydrolyze the cyclic zylpenicillin (procaine penicillin) is more rapidlyamide bond of the beta-lactam ring and inactivate absorbed and reaches higher plasma concentra-these antibiotics. Staphylococcal beta-lactamase is tions when injected into the muscles of the neckproduced by coagulase-positive staphylococci. muscles than those of the hindquarters and whenThis is the most clinically relevant beta-Iactamase. injected i.m. rather than by s.c. injection. BenzathineThe genes controlling the synthesis of these benzylpenicillin (benzathine penicillin) producesenzymes are encoded on plasmids and the the longest duration of penicillin concentrations,enzymes are exocellular. These enzymes typically but concentrations remain below the MIC of mostdo not inactivate cephalosporins and pathogens; consequently, it is of no therapeuticantistaphylococcal penicillins (e.g. cloxacillin, value and should not be used in equine practice.oxacillin). The beta-lactamase enzymes produced The absorption of penicillin in horses following p.o.by Gram-negative bacteria form a diverse group administration is too poor to be of practical use.that can be encoded on the chromosome and/or a Being weak acids with a pKa of 2.7/ penicillinsplasmid. Chromosome-mediated beta-lactamases have low Vd values (typically 0.2-Q.31/kg). Afterhydrolyze both penicillins and cephalosporins. absorption, penicillins distribute mainly into theE. coli beta-lactamase genes are transferred by ECE CSF concentrations are 10% of plasma con-plasmids and also hydrolyze both groups of drug. centrations unless the meninges are inflamed.Inhibitors such as clavulanic acid and sulbactam Penicillins distribute into milk but reach onlycan inactivate most beta-lactamases. subtherapeutic concentrations for most bacteria. Penicillins cross the placenta but are not associated with producing adverse effects in the fetus. ProteinPENICILLIN G (BENZYLPENICILLIN) binding is considered to be moderate (52-54%).Indications Elimination of the penicillins is primarily via the kidneys, by glomerular filtration and active tubu-Penicillin G (benzylpenicillin) remains the agent lar secretion. The half-life is approximately 1h afterof first choice for the treatment of streptococcal i.v. injection. Because of slow absorption from theinfections in horses and is, therefore, indicated injection site, the half-life of procaine benzylpeni-for the treatment of strangles, pneumonia and cillin (procaine penicillin) is approximately 7h.pleuropneumonia. Its efficacy may be limited bypoor drug penetration into abscesses, as is seenin strangles. Penicillin G is also indicated in the Drug interactions and adversetreatment of most anaerobic infections; however, effectsBacteroides (B.) fragilis is often resistant. Penicillin Concurrent administration with phenylbutazoneG is usually combined with other drugs to achieve increases plasma concentrations, but lowers tis-a broad spectrum of activity. Its activity may be sue concentrations, of penicillin G. Bacteriostaticsynergistic with the aminoglycosides and additive antimicrobial agents, such as chloramphenicol andwith the fluoroquinolones. the tetracyclines, antagonize the antibacterial activ- ity of penicillin G.Pharmacokinetics Immune-mediated reactions to penicillin G include anaphylaxis (a type I hypersensitivityThe sodium and potassium salts of penicillin G reaction), hemolytic anemia and thrombocytope-are the only formulations suitable for i.v, admin- nia (type II hypersensitivity reactions). Anaphy-istration. They are also the most rapidly absorbed lactic reactions can be fatal, so epinephrine
  • 27. 24 EQUINE CLINICAL PHARMACOLOGY(adrenaline) should be kept near at hand when penicillin that can lead to residues in food-administering penicillins. producing animals. Adverse CNS effects occur when the procaine Benzathine benzylpenicillin is a very insolubleportion of the procaine benzylpenicillin (procaine salt. It is administered i.m. in "long-acting" for-penicillin) formulation is given intravascularly. mulations that contain one-half procaine ben-The signs of toxicity include hyperexcitability, zylpenicillin (procaine penicillin) and one-halfmuscle tremors, ataxia, apnea and cardiac arrest. benzathine benzylpenicillin. Any clinical effect isThere is no specific treatment for procaine toxicity; from the procaine benzylpenicillin (procaineone can only attempt to prevent the horse from penicillin) portion of these formulations since theinjuring itself and others until the effects of the benzathine penicillin provides persistent but sub-procaine wear off. The CNS reaction can be therapeutic plasma concentrations.prevented by pretreatment with diazepam. Thesolubility of the procaine fraction of procainebenzylpenicillin (procaine penicillin) formulations AMINOPENICILLlNS: AMOXICILLINincreases with increasing ambient temperature, AND AMPICILLINso these products should be stored in a cool place Indicationsto reduce the risk of reactions. Procaine is a com-mon cause of positive drug tests in racehorses and The aminopenicillins are able to penetrate theother performance horses. Procaine benzylpeni- outer layer of Gram-negative bacteria better thancillin (procaine penicillin) should be avoided in penicillin G; they have activity against Gram-these animals. positive bacteria and many Gram-negative bacte- The sodium or potassium content of i.v, for- ria (E. coli and Salmonella and Pasteurella spp.).mulations can contribute to electrolyte imbalances Amoxicillin is superior to ampicillin in its abilityassociated with congestive heart failure and renal to penetrate Gram-negative bacteria. Resistancefunction impairment. Care should be taken when to the aminopenicillins is easily acquired by someusing these formulations in neonates. Gram-positive and Gram-negative bacteria via plasmids; therefore, these agents are not usually effective against Staph. aureus and Klebsiella, ProteusFormulations and Pseudomonas (Ps.) spp. Most anaerobes, exceptPenicillin G is available in injectable formulations. beta-lactamase-producing strains of BacteroidesThe sodium and potassium salts (crystalline peni- spp., are susceptible to the aminopenicillins.cillin) are water-soluble formulations and maybe injected i.v., i.m. or s.c. They rapidly produce Pharmacokineticshigh plasma concentrations but have very shorthalf-lives, so must be administered frequently. In horses, amoxicillin or ampicillin sodium are wellThe potassium salt must be administered more absorbed following i.m. or s.c. administration.carefully than the sodium salt, as rapid i.v, admin- Amoxicillin or ampicillin trihydrate are poorlyistration can cause cardiac arrhythmias. These soluble salts that are given i.m, and produce lowerformulations are frequently used for their Gram- plasma concentrations that extend over a longerpositive activity in life-threatening diseases and period of time. Amoxicillin and ampicillin are tooconditions such as surgical colic, neonatal sep- poorly absorbed following p.o. administration toticemia and clostridial myositis. be clinically useful in horses. The ampicillin esters, Procaine benzylpenicillin (procaine penicillin) pivampicillin and bacampicillin, show promiseis a poorly soluble salt that is absorbed slowly for p.o. use but are currently not available for usefollowing i.m, or s.c. injection. It is the most com- in horses.monly used formulation of penicillin in horses. The aminopenicillins are distributed rapidlyInjections s.c. cause severe local inflammation and widely into most tissues, except the eye andand hemorrhage, as well as leaving deposits of the accessory sex glands. They distribute poorly
  • 28. 2. ANTIMICROBIAL THERAPY 25into the CSF unless the meninges are inflamed. administered with the aminoglycosides. They areTheir penetration into synovial fluid is high. susceptible to beta-lactamase enzyme degradation.The V d of amoxicillin is 0.192I/kg in adult horses Extended-spectrum penicillins are of limited useand 0.2651/kg in foals. The Vd of ampicillin is in horses because of the expense of therapy.0.18I/kg in adult horses. Amoxicillin is moder- Ticarcillin is primarily used by intrauterineately protein bound (38%), while ampicillin is administration for the treatment of streptococcalminimally protein bound (6.8-8%). Following an and pseudomonad metritis (see Ch. 11) in mares.i.m. dose of ampicillin sodium at lOmg/kg, peak Both ticarcillin and carbenicillin are used for theserum concentrations of 6.2-9.7 J.1g/ml are reached treatment of infectious keratitis.in 16 min. Amoxicillin and ampicillin are excreted prima-rily unchanged in the urine. The half-life of amoxi- Pharmacokineticscillin is 0.5-1.5h in adults and 0.75h in I-week-old Although there is limited pharmacokinetic infor-foals. The half-life of ampicillin is 0.5-1.5h. mation available in horses, these agents appear to be similar to other penicillins. Following i.v. injection of ticarcillin to horses, at a dose rate ofDrug interactions and adverse 44 mg/kg, the serum concentration at 30 min waseffects 104.3J.1g/1 and the mean peak peritoneal fluidThe drug interactions and adverse effects of the concentration (61.4 J.1g/l) was reached 2 h afteraminopenicillins are the same as for penicillin G. injection. The half-life of ticarcillin was 0.94h.Ampicillin trihydrate may cause i.m. injection site Following i.m. injection (44 mg/kg), the peakreactions in horses: mild to moderate heat, pain serum (28.3J.1g/l) and peritoneal fluid (19.2J.1g/l)and/or swelling. concentrations were reached after 2 h. The bio- availability of ticarcillin was 64.9%.Formulations Drug interactions and adverseSodium ampicillin is approved for i.v, or i.m, effectsadministration to humans and horses. These for-mulations have a very short shelf-life after recon- Drug interactions and adverse effects of thestitution. The human approved formulation come extended-spectrum agents are the same as forin a variety of presentations that are more con- penicillin G.venient for use in different sizes of horses. Thetrihydrate salts of amoxicillin and ampicillin areavailable for i.m, administration to cattle. Formulations These drugs are available as sodium salts. In gen- eral, they are expensive and their use in equineEXTENDED-SPECTRUM practice is limited. A veterinary approved formu-PENICILLINS lation of ticarcillin is available in the USA forIndications intrauterine administration to mares.The extended-spectrum penicillins include car-benicillin and ticarcillin. Ticarcillinhas been shown CEPHALOSPORINSto be the most efficacious antimicrobial drug tested Indicationsversus Gram-positive bacteria, with 93% beingsusceptible in vitro. This group of penicillins can The widespread emergence of penicillin-resistantpenetrate the outer cell wall of Pseudomonas spp. staphylococci in the 1950s led to the developmentand other Gram-negative bacteria. They are active of the cephalosporins. The cephalosporins haveagainst anaerobes and are synergistic when the same mechanism of action as the penicillins
  • 29. 28 EQUINE CLINICAL PHARMACOLOGYbut are more resistant to bacterial defenses. The against Gram-negative bacteria that are resistantcephalosporins are usually grouped into three to first-generation agents (e.g. E. coli and Klebsiella,"generations" based primarily on their antibac- Proteus and Enterobacter spp.). Their increasedterial activity. However, some of the newer spectrum of activity results from increased resist-cephalosporins do not fit easily into this scheme. ance to Gram-negative beta-lactamases, Second-Cephalosporins are used widely for prophylaxis in generation cephalosporins are usually activecardiovascular, orthopedic, biliary and abdominal against anaerobes, and cefoxitin is effective againstsurgery. B. fragilis. The cost of these agents is roughly twice First-generation cephalosporins are effective that of the first-generation cephalosporins; there-against most Gram-positive cocci, including beta- fore, their use is limited to situations where thelactamase-producing staphylococci, by virtue of infection is resistant to a first-generation drug, sus-their higher affinity for PBP. They are very active ceptible to a second-generation drug and the onlyagainst staphylococci and streptococci, with the other therapeutic options produce unacceptableexception of MRSA and enterococci. Although side-effects.most corynebacteria are susceptible, R. (formerly Third-generation cephalosporins haveCorynebacterium) equi is usually resistant. They also increased activity against Gram-negative bacteriahave greater activity against Enterobacteriaceae because of their resistance to beta-lactamases.than other beta-lactam antibiotics, but may be In the past, they were considered to have unreli-degraded by Gram-negative beta-lactamases, mak- able and erratic effects against Gram-positiveing them ineffective against some Gram-negative organisms. However, good activity against Gram-bacteria. Among the commonly encountered positive organisms, except for enterococci andGram-negative bacteria, E. coli and Klebsiella, Listeria spp., has been demonstrated recently. SomeHaemophilus, Proteus, Actinobacillus, Pasteurella of the third-generation products (ceftazidimeand Salmonella spp. are usually susceptible to the and cefoperazone) have good activity againstfirst-generation cephalosporins. Indole-positive Pseudomonas spp. The use of the third-generationProteus, Enterobacter, Serratia and Pseudomonas spp. cephalosporins is usually restricted to bacterialare resistant. Most anaerobic bacteria are sus- infections caused by multiple drug-resistantceptible, with the exception of beta-lactamase- strains. These drugs are most likely to be usefulproducing Bacteroides spp. and Clostridium difficile. in equines with neonatal septicemia, nosocomial First-generation cephalosporins are an effective pneumonia, postoperative wound infections andalternative to penicillins. Only 3-7% of humans urinary tract infections related to catheterization.allergic to penicillin are allergic to cephalosporins; Cefotaxime has been investigated in the treat-therefore, they may usually be used in penicillin- ment of meningitis and septicemia in neonatalsensitive individuals. First-generation cephalos- foals. All of the third-generation cephalosporinsporins do not cross the blood-brain barrier. All of are extremely expensive and are, therefore, rarelythe first-generation cephalosporins have essen- used in veterinary medicine.tially the same spectrum of activity, so the choice Ceftiofur is marketed as a "new" generationof an individual drug is usually based on phar- cephalosporin as it does not clearly fall into themacokinetic and economic considerations. The previous classification scheme. It is currentlyfirst-generation cephalosporins used most com- approved for the treatment of streptococcal infec-monly in horses are cefalotin and cefazolin. tions in horses. It has broader Gram-positiveCefazolin is promoted as being better tolerated spectrum of activity than the third-generationafter i.m. injection than the other first-generation cephalosporins and has activity against anaerobes.cephalosporins. Ceftiofur has no activity against Pseudomonas spp. Second-generation cephalosporins are no more and does not penetrate into the CNS. It is meta-active against Gram-positive bacteria and may bolized very rapidly in vivo to its active metabo-be less active against staphylococci than the lite desfuroylceftiofur. Desfuroylceftiofur is veryfirst-generation drugs but have greater activity highly protein bound because of a sulfhydryl
  • 30. 2. ANTIMICROBIAL THERAPY 27group in its chemical structure. The efficacy of Most cephalosporins are excreted unchanged inceftiofur is attributed to desfuroylceftiofur bind- the urine. Renal elimination of the cephalosporinsing to acute phase proteins, such as oq-antitrypsin, occurs through a combination of glomerular fil-that act as a reservoir of active drug and carry it tration and active tubular secretion. The dosageto sites of inflammation. Because of the high degree regimen for most cephalosporins must be modi-of protein binding, ceftiofur does not readily cross fied in renal failure. Cefalotin, cefapirin, cefotaximeinto the milk when administered parenterally. and ceftiofur are deacetylated by the liver. TheirDesfuroylceftiofur does not have the same activ- metabolites have significant antibacterial activity.ity as ceftiofur against certain pathogens, so the For the cephalosporins that undergo hepaticresults of susceptibility tests using ceftiofur disks metabolism, hepatic insufficiency may result inmay be misleading: Staph. aureus is four to eight decreased metabolism and increased drug accu-times less sensitive to desfuroylceftiofur than to mulation. Most cephalosporins are eliminatedceftiofur; Proteus mirabilis has widely variable rapidly after systemic administration; with half-susceptibility to desfuroylceftiofur. lives of 0.25-1 h. Ceftiofur has a relatively long half-life (3-5 h).Pharmacokinetics Drug interactions and adverseThe absorption of the cephalosporins is rapid fol- effectslowing i.m, or s.c, administration. The extent ofabsorption depends on the drug used and the In general, the cephalosporins have a high thera-species treated. The absorption of cephalosporins peutic index. Most of the adverse effects producedis erratic following p.o. administration to horses. are the same as those reported for the penicillins. Cephalosporins distribute into most body flu- In humans, there is a low incidence of cross-ids and tissues, including pleural fluid, synovial sensitivity with the penicillins for type I and type IIfluid, pericardial fluid and urine. Cephalosporins hypersensitivity reactions. Bleeding disorders,distribute into milk, but therapeutic concentra- caused by vitamin K antagonism, are associatedtions are not reached following systemic admin- with some of the cephalosporins used in humansistration at accepted dose rates. Cephalosporins but have not been reported in animals. Anemiacross the placenta but do not appear to cause and thrombocytopenia develop in dogs adminis-adverse effects in the fetus. Their penetration into tered high doses of ceftiofur for an extendedcortical and cancellous bone is usually adequate. period. Horses may develop diarrhea after treat-Most cephalosporins penetrate poorly into the ment with ceftiofur. The currently availableaqueous humor, accessory sex glands and CSF. cephalosporins are considered to be potentiallyCephalosporins have typically low Vd values in nephrotoxic, either via the deposition of immunehorses: 0.19I/kg for cefazolin, 0.15I/kg for cefa- complexes in the glomerular basement membranelotin, 0.17I/kg for cefapirin, O.4I/kg for cefradine or as a direct effect that leads to acute tubularand 0.121/kg for cefoxitin. necrosis. However, animal studies have shown Ceftiofur and its active metabolite desfuroyl- that the cephalosporins protect against nephro-ceftiofur distribute differently to the other toxicity. In human medicine, it is recommendedcephalosporins because of their high degree of that cephalosporins not be used in conjunctionprotein binding. Reversible covalent bonding with with aminoglycosides.plasma and tissue proteins produces lower thanexpected free concentrations of ceftiofur and des- Formulationsfuroylceftiofur following the administration ofclinically effective doses. Tissue chamber studies Cefazolin, cefotaxime, cefoxitin, cefalotin, cefapirinhave shown that concentrations of parent drug and ceftiofur are all available as sodium salts forand active metabolite are higher in infected than injection. Ceftiofur is the only approved veterinaryin normal tissues. formulation.
  • 31. 28 EQUINE CLINICAL PHARMACOLOGYIMIPENEM linkage. Streptomycin was originally developed for the treatment of tuberculosis in humans (itsThe carbapenem antibiotics such as Imlpenem discoverer was awarded a Nobel Prize), but thewere developed to deal with the beta-lactarnase- newer aminoglycosides such as gentamicinproducing Gram-negative organisms resistant to and tobramycin have been targeted to treatthe penicillins. Pseudomonas spp. infections. In veterinary medi- cine, these agents are important in the treatmentIndications of Gram-negative infections caused by entericImipenem is a carbapenem antibiotic. It has the pathogens such as E. coli.broadest spectrum of activity of all antibiotics.It is the most potent of the newer agents against Mechanism of actionGram-positive cocci and anaerobes and morethan 90% of Gram-negative organisms including Aminoglycosides must penetrate bacteria tothose resistant to other beta-lactam antibiotics assert their effects. Susceptible, aerobic Gram-and the aminoglycosides, are susceptible. Imipe- negative bacteria actively pump aminoglycosidenem is highly resistant to most beta-Iactarnases. into their cells. This is initiated by an oxygen-Resistance to imipenem develops rapidly in Pseu- dependent interaction of the antibiotic cationsdomonas aeruginosa through changes in its outer and the negatively charged ions of the bacterialmembrane proteins that reduce permeability. membrane lipopolysaccharides. This interactionImipenem is rarely used in veterinary medicine affects membrane permeability by displacingbecause of the expense of therapy. divalent cations (calcium, magnesium). Once inside the bacterial cell, aminoglycosides bind toPharmacokinetics the 30S ribosomal subunit and cause misreading of the genetic code, thus interrupting normal bac-Pharmacokinetic studies have not been performed terial protein synthesis. This leads to changes inin horses; however, i.v. doses of 0.7-1.1 mg/kg cell membrane permeability, resulting in addi-three times a day have been suggested as being tional antibiotic uptake, further cell disruptionsuitable for use in small animals. Imipenem has and ultimately cell death.to be administered i.v, because it is not absorbed Aminoglycosides are bactericidal and producefollowing p.o. administration. Imipenem has been dose- (concentration-) dependent effects. For exam-shown to penetrate inflamed meninges. It is ple, gentamicin concentrations of 0.5-5.0 fLg/mlmetabolized extensively by the renal tubules to a are bactericidal in Gram-positive and somepotentially toxic compound. Therefore, it is usu- Gram-negative bacteria. Gentamicin concentra-ally combined with cilastatin, a drug that inhibits tions of 10-15 fLg/ml are effective against morethe renal tubular enzymes. The combined prod- resistant bacteria such as Pseudomonas, Klebsiellauct produces high urine concentrations of active and Proteus spp. The clinical implication is that highantibiotic and avoids renal toxicity. In the presence initial doses of aminoglycosides increase ionicof cilastatin, 70%of a dose of imipenem is excreted bonding, which enhances the initial, rapid, con-unchanged in the urine. The half-life of imipenem centration-dependent phase of antibiotic internal-in the dog is 30-45 min. ization and leads to greater immediate bactericidal activity. Clinical studies in humans have demon- strated that proper initial therapeutic doses ofAMINOGLYCOSIDE ANTIBIOTICS aminoglycosides are critical in reducing the mor- tality from Gram-negative septicemia.The aminoglycoside antibiotics include strep- The aminoglycosides are effective againsttomycin, neomycin, gentamicin, amikacin, most Gram-negative bacteria, includingtobramycin and kanamycin. They have a chemical Pseudomonas spp. They are usually effective againststructure of amino sugars joined by a glycoside staphylococci, although resistance can occur. They
  • 32. 2. ANTIMICROBIAL THERAPY 28are often effective against enterococci, but their variants with altered metabolism and impairedaction against streptococci is more effective when aminoglycoside uptake. This resistance developscombined with a beta-lactam antibiotic. Salmonella within 2h of exposure to the aminoglycosideand Brucella spp. are intracellular pathogens that but it reverses if the drug is absent for a periodare often resistant because of poor intracellular (usually hours). If the aminoglycoside concentra-drug penetration. Aminoglycosides are ineffec- tion remains constant, such as during a constanttive against anaerobic bacteria because bacterial i.v. infusion, adaptive resistance persists andpenetration is oxygen dependent. The amino- increases. Therefore, intermittent dosing regimensglycosides induce significant PAEs. The duration are most effective for aminoglycoside therapy.of these PAEs tends to increase as the initial drugconcentration increases. Pharmacokinetics The antimicrobial activity of aminoglycosidesis enhanced in an alkaline environment (pH 6-8). The aminoglycosides are well absorbed followingThey also bind to and are inactivated by the nucleic i.m, and s.c. administration because of large poresacid material released by decaying white blood in the capillary walls of the subcutaneous tissuescells. They are, therefore, usually ineffective in the and muscle and the bioavailability approachesacidic, hyperosmolar, anaerobic environment of 100%. Aminoglycosides are absorbed poorly afterabscesses. p.o. administration but enough may be absorbed in animals with enteritis to result in drug residues in food-producing animals.Resistance mechanisms The aminoglycosides are large polar drug mol-Bacteria have a number of mechanisms that confer ecules that remain primarily in the ECF space;resistance to the aminoglycosides. Penetration of they are highly ionized at physiological pH. The Vdthe bacterial cell wall is essential for antimicrobial of aminoglycosides is higher in neonates, becauseactivity. Strict anaerobes respire without a func- of their increased ECF volume, than in adulttioning electron-transport system and are unable horses. Therefore, to achieve the same plasma con-to drive the energy-requiring phases of amino- centrations, a foal must be given a higher doseglycoside uptake. Exposure to the aminoglyco- than an adult horse. Following parenteral admin-sides results in the development of bacterial istration, effective concentrations are obtainedmutants with altered cell membrane structure that in synovial, perilymph, pleural, peritoneal andprevents such penetration. Bacterial enzymes have pericardial fluids. Therapeutic concentrations arebeen identified that attack different parts of the not achieved in bile, milk, CSF, respiratory secre-aminoglycoside molecule. The members of the tions, accessory sex glands and ocular fluids.aminoglycoside group vary in their susceptibility Aminoglycosides readily cross the placenta andto these enzymes and thus cross-resistance is not may be hazardous to fetal kidneys. The proteinuniform. Production of these enzymes by bacte- binding of the arninoglycosides is low.ria is plasmid encoded and so is transferable both Aminoglycosides are almost exclusively elimi-within and between bacterial strains. Amikacin is nated by glomerular filtration. Elimination isthe least susceptible to enzymatic inactivation. dependent on cardiovascular and renal function, Aminoglycoside antibiotics also have a unique age, fever, other physiological factors and the Vd.method of bacterial resistance known as first- The half-lives are usually I-2h in normal adultexposure adaptive resistance. Both inhibitory and horses but are increased in horses with renal dys-subinhibitory concentrations of aminoglycosides function and in neonates. Increased dosage inter-can result in resistant bacterial cells, which survive vals must be used in these patients to preventthe initial ionic bonding. This adaptive resistance nephrotoxicity. The renal elimination of the amino-results from decreased transport of aminoglyco- glycosides increases with age. The half-life ofsides into the bacteria. Exposure to one dose of an gentamicin is approximately 50% longer inaminoglycoside is sufficient to produce resistant I-day-old than in 30-day-old foals.
  • 33. 30 EQUINE CLINICAL PHARMACOLOGY cells, disrupting other organelles and causing cellDrug interactions and adverse death.effects Approximately 10-26% of human patientsAminoglycosides are inactivated when combined reportedly develop nephrotoxicosis associatedin vitro with many other drugs because of pH with aminoglycoside therapy. Individual cases ofincompatibilities. Aminoglycosides exhibit syn- arninoglycoside nephrotoxicity have been reportedergism with beta-lactam antibiotics against strep- in horses but the incidence is not known.tococci, enterococci, Pseudomonas spp. and other There are several risk factors for aminoglyco-Gram-negative bacteria. Ticarcillin binds amino- side nephrotoxicity.glycoside molecules in vivo and may decreaseaminoglycoside toxicity after an accidental over- 1. Prolonged therapy. Nephrotoxicity isdose. Supplemental iron increases the risk of associated with prolonged therapy (longer thannephrotoxicity and ototoxicity. Halothane anes- 7-10 days), which allows substantial amountsthesia reduces the Vd and increases the half-life of drug to accumulate within the renal tubularof gentamicin. Concurrent administration of gen- cells. Overdosing within the first 1-2 days oftamicin and phenylbutazone reduces the Vd of aminoglycoside therapy does not lead togentamicin by 26% and the half-life by 23%. These nephrotoxicity provided that renal function isalterations are thought to be caused by increased within normal limits.entry of gentamicin into tissues; concurrent 2. Metabolic acidosis and electrolytephenylbutazone therapy may improve the effi- disturbances. Acidic environments increase thecacy of gentamicin in treating Gram-negative ionization of aminoglycosides and positivetissue infections. Fluid administration i.v, does cations compete with the aminoglycosides fornot change the pharmacokinetics of concurrently binding sites. Consequently, metabolic acidosisadministered aminoglycosides. Coadministration and electrolyte imbalances such as hypokalemiaof aminoglycosides and other nephrotoxic drugs and hyponatremia increase the binding ofshould be avoided. aminoglycosides to the phospholipid receptors The nephrotoxicity caused by aminoglyco- on the renal tubular cells, increasingsides (acute tubular necrosis) is of great concern intracellular accumulation.and limits the practical use of these agents. The 3. Volume depletion. Dehydration, sodiumaminoglycosides enter the renal tubule after fil- depletion, shock, endotoxemia and diuretictration through the glomerulus. A small amount drugs decrease the ECF volume, therebyof drug binds to phospholipid receptors on the increasing aminoglycoside concentration andluminal surface of the proximal tubular cells via a necessitating changes in drug dose.charge-mediated process. The phospholipid recep- 4. Concurrent nephrotoxic drug therapy.tor is a common anionic binding site that is com- Concurrent therapy with other nephrotoxicpeted for with amino acids, cationic polypeptides drugs, such as non-steroidal anti-inflammatoryand electrolytes. The aminoglycoside is taken drugs (NSAlDs) and amphotericin B, potentiatesinto the cells via carrier-mediated pinocytosis. the nephrotoxicity of the aminoglycosides.The drug is translocated into cytoplasmic vac- 5. Pre-existing renal disease. Arninoglycosidesuoles, which fuse with lysosomes. The aminogly- are eliminated by renal excretion and a decreasecoside is sequestered unchanged in the lysosomes. in renal function prolongs the persistence of theseWith continued pinocytosis, the drug continues nephrotoxic drugs. The glomerular filtration rateto accumulate within the lysosomes. The accu- decreases with age, as a result of progressivemulated aminoglycoside interferes with normal vascular changes in the glomeruli; therefore, renallysosomal function and eventually overloaded excretion may be reduced in aged horses.lysosomes swell and rupture. Lysosomal enzy- 6. Elevated trough plasma concentrations.mes, phospholipid and the aminoglycoside are The uptake of the aminoglycosides by the renalreleased into the cytosol of the proximal tubular tubular cells is a saturable process. Sustained
  • 34. 2. ANTIMICROBIAL THERAPY 31high trough plasma concentrations result in Therapeutic drug monitoring, to optimize peakgreater drug accumulation and nephrotoxicosis. and trough drug concentrations and monitorDosage regimens that result in high peak changes in the drug elimination rate, can be usedconcentrations and low to undetectable trough to prevent aminoglycoside nephrotoxicity. If this isconcentrations reduce the risk of nephrotoxicity. not available, then once daily, high-dose therapy is recommended. The development of nephro- High-protein diets protect against the develop- toxicity can initially be identified by increases in ment of aminoglycoside nephrotoxicity. Increased urine gamma-glutamyltransferase (GGT) and dietary protein increases the glomerular filtration I3-N-acetylglucosaminidase (AGS) enzymes and rate and renal blood flow, promoting aminogly- increases in the urine GGT:creatinine and urinecoside excretion. Dietary proteins also compete AGS:creatinine ratios. Urinary GGT and AGSwith the aminoglycosides for the phospholipid activity increases within 5 days of aminoglyco-receptors on the renal tubular cells. Increased side administration and provides an earlier indi-dietary protein also increases the Vd of the amino- cation of renal tubular disease than any otherglycosides, increasing tissue concentrations while indices in clinical pathology. Elevations of blooddecreasing plasma concentrations. Horses fed urea nitrogen and serum creatinine concentra-an alfalfa diet have a significantly lower degree of tions are not typically seen for at least 6 days aftergentamicin-induced nephrotoxicity than horses starting aminoglycoside therapy. Proteinuria,fed an oat grain diet. Calcium supplementation urine casts and a decrease in urine specific gravity(20mg/kg calcium gluconate i.v, three times a all occur after approximately 1 week of amino-day) also decreases aminoglycoside nephrotoxi- glycoside therapy.city through cation competition for the renal Aminoglycosides should be used cautiously inbinding sites. horses with endotoxemia. Even a low serum Clinically, aminoglycoside nephrotoxicity endotoxin concentration may increase toxicity bymost commonly manifests as an asymptomatic increasing kidney aminoglycoside concentrations.rise in serum creatinine concentrations 7-10 days The bactericidal action of the aminoglycosidesafter the initiation of treatment. Acute renal fail- against Gram-negative bacteria may transientlyure is generally not oliguric; oliguria is a poor increase endotoxin concentrations. Bacterial tox-prognostic sign. Significant tubular dysfunction ins may have local synergism with the aminogly-actually precedes the decline in the glomerular cosides in damaging the renal tubular cells.filtration rate. Increased urinary excretion of lyso- Administration of a NSAID prior to aminoglyco-somal enzymes and 132-microglobulin, decreased side administration may prevent the deleteriousreabsorption of potassium and magnesium ions effects of endotoxin.and tubular resistance to vasopressin (antidi- Aminoglycoside ototoxicity is a significanturetic hormone), resulting in polyuria and partial problem in human medicine but has not beennephrogenic diabetes insipidus, can also be seen. investigated in horses. Aminoglycosides accumu-Nephrotoxicity is usually reversible on discontin- late in the tissues of the inner ear by binding to auing the drug treatment; however, renal dys- phospholipid receptor. Streptomycin, tobramycinfunction may persist for some time as a result of and gentamicin damage the cochlear divisionrenal cortical drug accumulation. Nephrotoxicity of the 8th cranial nerve, resulting in vertigo.increases the half-lives of the aminoglycosides to Dihydrostreptomycin, amikacin, kanamycin and24-45 h in horses. Peritoneal dialysis is useful for neomycin damage the auditory division of the 8thlowering creatinine and blood urea nitrogen con- cranial nerve, resulting in deafness.centrations, but it does not effectively increase the Neuromuscular blockade is a rare side-effect ofclearance of the accumulated aminoglycoside. If the aminoglycosides, related to blockade of acetyl-there is enough healthy renal tissue remaining, choline at the nicotinic cholinergic receptor. Thisregeneration and hypertrophy of nephrons will is most often seen as respiratory depression andrestore normal renal function. apnea when anesthetic agents are administered
  • 35. 32 EQUINE CLINICAL PHARMACOLOGYconcurrently with aminoglycosides. Postsynaptic effective against Proteus and Klebsiella spp., E. coliblockade can be reversed using a cholinesterase and staphylococci. It has only moderate activityinhibitor such as neostigmine. Presynaptic block- against streptococci and poor activity againstade can be antagonized by calcium ions, admin- anaerobes. Transmissible resistance by Gram-istered i.v. negative bacteria occurs in proportion to gentam- Amikacin, as a 2g intrauterine dose, was not icin use. Bacteria that are resistant to gentamicinfound to impair fertility when administered to are also resistant to neomycin, streptomycin andmares 8 h prior to breeding. Mares should not be kanamycin. The reverse is not true, as manybred within 8 h of intrauterine treatment with strains resistant to the other aminoglycosides willamikacin and gentamicin should not be adminis- remain susceptible to gentamicin. Gentamicin istered to these mares on the day of breeding. approved only for intrauterine administration to horses. However, it is often used parenterally inSTREPTOMYCINI the therapy of Gram-negative septicemia andDIHYDROSTREPTOMYCIN infectious arthritis and prophylactically before abdominal surgery.Dihydrostreptomycin was developed as a non-ototoxic alternative to streptomycin, however itwas just as ototoxic. Clinically, dihydrostrepto- Pharmacokineticsmycin used to be given in combination with peni- In adult horses, the Vd of gentamicin ranges fromcillin G, but most of these injectable products are 0.121/kg to 0.2411kg. In foals, it is approximatelyno longer available. 0.311kg and this decreases to adult values by 6 months of age. The predominant site of drugNEOMYCIN accumulation is the renal cortex. When repeated doses of gentamicin are given, tissue concentra-Neomycin is not usually administered parenter- tions (from highest to lowest) are found in the renalally to animals because of nephrotoxicity and oto- cortex, renal medulla, liver /lung/ spleen andtoxicity. Only 3%of a dose of neomycin is absorbed skeletal muscle. Gentamicin is distributed rapidlyfollowing p.o. administration; it is, therefore, used into synovial fluid in normal horses and 2 h afterin the therapy of coliform enteritis in small and an i.v, dose, at a dose rate of 4.4mg/kg, reaches alarge animals. It is available as tablets, boluses peak concentration of 6.4 fig/ ml. Local inflam-and water additives, in many different combina- mation and repeated dosing may further increasetions with antibiotics, corticosteroids and anti- synovial fluid concentrations. After intraarticularcholinergic agents. It can also be used to decrease administration of gentamicin (150 mg), peak syn-nitrogenous waste production by the normal ovial concentrations of approximately 2 mg/ ml aregastrointestinal flora in animals with hepatic reached within 15min. Gentamicin (2.5g) admin-encephalopathy. Neomycin is not absorbed istered once daily for 5 days by the intrauterinethrough the skin, so it is frequently utilized as the rou te produced concentrations of 42 fig / ml inantibacterial constituent in ophthalmic formula- endometrial tissue 24 h after the last dose. The pro-tions (especially in combination with bacitracin tein binding of gentamicin is <30%.and polymyxin B) and in preparations for the In horses, 75-100% of a gentamicin dose istreatment of otitis externa in small animals. excreted unchanged in the urine in the first 8-24 h after administration. The half-life is 1-2h and isGENTAMICIN longer in neonatal foals than in older foals and adult horses. Any gentamicin that accumulates inIndications the renal cortical tissue is eliminated slowly; how-Gentamicin is more active against a wide range ever, these levels are often below the limits of quan-of bacteria, particularly against Pseudomonas spp. tification of the assays used and are, therefore, notthan the earlier aminoglycosides. It is also very demonstrated in pharmacokinetic studies.
  • 36. 2. ANTIMICROBIAL THERAPY 33Therapeutic drug monitoring were reached within 1 h of intrauterine adminis- tration of 2g amikacin. Regional i.v. perfusionThe gentamicin dosage regimen should be deter- of amikacin (125mg) produced sufficiently highmined using therapeutic drug monitoring so as concentrations to be effective in the treatment ofto maximize efficacy and minimize toxicity. most equine pathogens in the joint fluid, bonePlasma concentrations of gentamicin differ widely and serum of the treated limb.between individual horses treated with the same The elimination of amikacin is similar to gen-dosage regimen. This variability in the relationship tamicin. The half-life of amikacin ranges frombetween the dose and plasma drug concentrations, 1 to 3 h in adult horses but may be as long as 5 hcombined with the narrow therapeutic range of the in neonatal foals.aminoglycosides, makes the monitoring of plasmaconcentrations in clinical cases very desirable.The desired peak plasma concentration is eight to Therapeutic drug monitoringten times the MIC of the suspected pathogen. The amikacin dosing regimen is best determinedWith once daily dosing, peak concentrations can using therapeutic drug monitoring. The sug-be measured in plasma 1-2 h after dosing. The gested peak concentration of amikacin (l h afterdesired trough concentration is <2 f..Lg/ml and it administration) is 25 f..Lg/ml and the trough con-is thought that the longer the period in the dosing centration should be <5 f.Lg/ml.interval that plasma concentrations are <2 ug/rnl,the lower the risk of nephrotoxicity. FormulationsFormulations Amikacin sulfate is approved for i.m. or S.c. injec- tion in horses (50 or 250mg/ml solution) and forGentamicin sulfate is approved for intrauterine intrauterine infusion (250mg/ml solution). Otheruse in mares. Other unapproved routes of admin- routes of administration that are not within theistration (i.v., i.m., s.c, and intraarticular routes) label indications (i.v. and intraarticular) are usedare used frequently. An ophthalmic solution of frequently.gentamicin is also approved for veterinary use. TOBRAMYCINAMIKACIN Tobramycin is structurally related to kanamycinIndications and has four times the activity of gentamicinAmikacin is derived from kanamycin and has the against Pseudomonas spp. It is an extremely expen-broadest spectrum of activity of the aminoglyco- sive drug. Its use in animals is usually limited tosides. It is less susceptible to bacterial enzyme the topical treatment of melting corneal ulcersinactivation than the other aminoglycosides, so it caused by gentamicin-resistant Pseudomonas spp.is usually reserved for therapy of gentamicin- using the ophthalmic solution approved for useresistant bacterial infections. in humans.PharmacokineticsThe V d of amikacin is 0.14-0.221/kg in adult horses CHLORAMPHENICOL ANDand 0.4-0.61/kg in foals. The distribution into tis- FLORFENICOLsues is similar to that for gentamicin. One hourafter i.v, administration of gentamicin (6.6mg/kg) Indicationspeak concentrations were 14 f..Lg/ml in peritoneal Chloramphenicol was isolated in 1947 from a soilfluid and 17 f.Lg/ml in synovial fluid. Concentra- actinomycete from Venezuela. Florfenicol is ations of greater than 40 f.Lgl g endometrial tissue fluorinated derivative of chloramphenicol. These
  • 37. 34 EQUINE CLINICAL PHARMACOLOGYantibiotics have a very broad spectrum of activity, levels are attained in the liver and kidneys, butbeing active against streptococci, staphylococci, therapeutic drug concentrations are attained inHaemophilus, Salmonella, Pasteurella and Brucella most tissues and fluids, including the ocularspp., anaerobes and Mycoplasma spp. Florfenicol humors and synovial fluid. Chloramphenicol mayhas activity against chloramphenicol-resistant achieve CSFconcentrations of up to 50%of plasmastrains of Staph. aureus, E. coli and Klebsiella, Proteus concentrations, when the meninges are normal, orand Salmonella spp. These agents are also active higher, when inflammation is present. Florfenicolagainst Rickettsia, Chlamydia and Hemobartonella does not penetrate the blood-brain barrier asspp. The use of chloramphenicol in food-producing readily as chloramphenicol but may reach thera-animals is prohibited because of its association peutic concentrations for some sensitive pathogenswith idiosyncratic aplastic anemia in humans. It in the CSF. The V d of chloramphenicol is 1.411/kgis used in horses to treat a variety of bacterial in adult horses and 1.6l/kg in neonatal foals.infections, especially where penetration into the The V d of florfenicol is O.72l/kg in adult horses.CNS or sequestered infections is desired. After a single i.m. administration of a long-acting florfenicol preparation to horses at a dose rate of 20mg/kg, peak plasma concentrations ofMechanism of action 1-2/log/ml were reached after 3h and plasmaChloramphenicol and florfenicol act by binding concentrations were still >0.25/log/ml after 72h.to the 50S ribosomal subunit and blocking trans- Chloramphenicol and florfenicol undergo hepa-fer RNA, inhibiting bacterial protein synthesis. tic metabolism (glucuronide conjugation) followedThey are considered bacteriostatic, but the MBCs by active renal tubular secretion. Only 5-15% of theof florfenicol for some respiratory pathogens are dose is excreted unchanged (glomerular filtration)low enough to consider it bactericidal. in urine. The half-life of chloramphenicol and flor- fenicol are <1 hand 1.8h, respectively, following i.v, administration to horses.Resistance mechanismsBacterial resistance to chloramphenicol occurs Drug interactions and adversefrom plasmid-mediated bacterial production of effectsacetylase enzymes. Acetylation of hydroxyl groupsprevents the drug binding to the 50S ribosomal Chloramphenicol is a hepatic microsomal enzymesubunit. There is less bacterial resistance to flor- inhibitor. It decreases the clearance of other drugsfenicol because of the substitution of a fluorine that are metabolized by the same enzymes (e.g.molecule for one of the hydroxyl groups. phenytoin, phenobarbital, pentobarbital and cyclophosphamide). It is not known if florfenicol has this effect. Chloramphenicol also profoundlyPharmacokinetics prolongs the duration of barbiturate anesthesia.Chloramphenicol is not administered i.v, to horses Therapeutic drug monitoring should be carriedbecause of its short half-life, which precludes out when any of these drugs are used concur-achieving therapeutic plasma concentrations. rently with chloramphenicol. ChloramphenicolInjections of chloramphenicol i.m. are associated may suppress antibody production if givenwith severe pain in horses and are not recom- prior to an antigenic stimulus and may affectmended. Chloramphenicol and florfenicol are the response to vaccination. Chloramphenicol andabsorbed rapidly and extensively after p.o. admin- florfenicol should not be administered concur-istration to horses. The bioavailability was 83%after rently with the penicillins and aminoglycosidesp.o. administration of chloramphenicol to neonatal (they may antagonize the activity of the penicillinsfoals and of florfenicol in an organic solvent. and aminoglycosides), fluoroquinolones (they Chloramphenicol and florfenicol distribute inhibit protein synthesis and thus interfere withwidely throughout the body. The highest drug the production of autolysins responsible for cell
  • 38. 2. ANTIMICROBIAL THERAPY 36lysis after the fluoroquinolones interfere with (propylene glycol, polyethylene glycol andDNA supercoiling) or macrolides (they act at the 2-pyrrolidone) that make it unsuitable for i.v.same site as the macrolides). administration. This product causes minor pain Idiosyncratic aplastic anemia occurs only in and swelling following administration into thehumans exposed to chloramphenicol. The reac- neck muscles in horses.tion is rare (1 in 30000) and not dose related. Thetoxic effects are related to the presence of thepara-nitro group on the chloramphenicol mole- POTENTIATED SULFONAMIDEScule. Florfenicollacks this group and is not asso-ciated with aplastic anemia in any species. IndicationsLong-term chloramphenicol therapy (> 14 days)is associated with dose-related anemia and pan- The sulfonamides are a group of organic com-cytopenia through a decrease in protein synthesis pounds with chemotherapeutic activity; they arein the bone marrow, especially in cats. Florfenicol antimicrobial agents and not antibiotics. They havemay cause similar reversible suppression of the a cornmon chemical nucleus that is closely relatedmyeloid series in bone marrow, but this does not to PABA, an essential component in the folic acidappear to be clinically significant during short- pathway of nucleic acid synthesis. The sulfon-term treatment regimens. amides are synergistic with the diaminopyrim- Florfenicol is associated with producing tran- idines, which inhibit an essential step further alongsient diarrhea in calves and has been associated the folate pathway. The combination of a sulfon-anecdotally with severe colitis in horses. In one amide and a diaminopyrimidine is advantageouspharrnacokinetic study, all of the horses and ponies because it is relatively non-toxic to mammaliandeveloped mild diarrhea following i.v., i.m, or cells (less sulfonamide is administered) and is lessp.o. administration of florfenicol. In a recent likely to select for resistant bacteria. Only thesestudy where the commercial florfenicol formula- so-called potentiated sulfonamides are used intion was administered i.m. to horses at 20mg/kg equine medicine. These drugs are formulated in aevery 48 h, all of the horses remained clinically ratio of one part diaminopyrimidine to five partsnormal but showed significant changes in com- sulfonamide, but the optimal antimicrobial ratiomensal fecal flora. High numbers of Clostridium at the tissue level is 1:20,which is achieved becauseperfringens and Salmonella spp. were isolated from the diarninopyrimidines are excreted more rapidlysome of the treated horses. Florfenicol should be than the sulfonamides.used with caution in horses because of its poten- Potentiated sulfonamides are usually activetial to induce antimicrobial-associated colitis. against streptococci, staphylococci, C. perfringens and some Fusobacterium and Bacteroides spp., Nocardia spp. and some Gram-negative bacteria,Formulations including strains of E. coli and Shigella, Salmonella,A water-soluble formulation of chloramphenicol Klebsiella, Pasteurella and Proteus spp. They are con-sodium succinate is available for i.v. use. Chlo- sidered ineffective against most obligate anaerobesramphenicol succinate is hydrolyzed to chloram- and should not be used to treat serious anaerobicphenicol in the liver. Chloramphenicol palmitate infections. Trimethoprim and pyrimethamine areis an insoluble ester suitable for p.o. administra- diaminopyrimidines that are used most commonlytion. Chloramphenicol palmitate is hydrolyzed in combination with the sulfonamides and theto chloramphenicol in the gastrointestinal tract. MIC of pathogens is generally lowered in suchChloramphenicol (generic and veterinary labeled) a combination.is also available in a variety of tablet and capsule Pyrimethamine is an aminopyrimidine agentstrengths. A long-acting formulation of florfenicol that is structurally related to trimethoprim but isis approved for i.m. and s.c, administration to more active against protozoa than bacteria. It iscattle. This formulation contains three carriers most effective against protozoa when administered
  • 39. 38 EQUINE CLINICAL PHARMACOLOGYin conjunction with a sulfonamide. In humans, Resistance to the diaminopyrimidines usuallypyrimethamine is used in the treatment of toxo- occurs by plasmid-encoded production ofplasmosis and malaria. In horses, it is used in com- diaminopyrimidine-resistant DHFR. Excessivebination with a sulfonamide drug in the treatment bacterial production of DHFR and a reduction inof equine protozoal myeloencephalitis (EPM) the ability of the drug to penetrate the bacterialcaused by Sarcocystis neurona (see Chs 3 and 9). cell wall also results in resistance. There is less resistance to the potentiated sulfonamides than to the individual agents.Mechanism of actionIn the folic acid pathway, the sulfonamides inhibit Pharmacokineticsthe bacterial enzyme dihydropteroate synthetase In general, the sulfonamides are readily absorbed(DPS), thereby blocking bacterial nucleic acid from the gastrointestinal tract of non-ruminants.synthesis. Sulfonamides substitute for PABA, Absorption may be delayed when the potentiatedpreventing its conversion into dihydrofolic acid. sulfonamides are administered with feed. InitialThis action is considered bacteriostatic. Since the serum concentrations are lower in a fed horseantimicrobial activity is by competitive substitu- than a fasted horse but the food effect is greatlytion, tissue concentrations of sulfonamides must be reduced by the third treatment day. The bioavail-high enough to prevent bacterial access to PABA. ability of one formulation of pyrimethamine isSulfonamides are ineffective in pus and necrotic 56% following p.o. administration.tissue where there are additional sources of PABA. Sulfonamides are weak acids. They distributeThey are safe to mammalian cells because mam- well but relatively slowly (compared withmalian cells do not require PABA and synthesize trimethoprim), and tissue concentrations are lowerdihydrofolic acid from dietary folate. Trimetho- than plasma concentrations. Some sulfonamidesprim and pyrimethamine inhibit bacterial folic-acid reach significant concentrations in the CSF. Thesynthesis at the step subsequent to that affected highest drug concentrations are in the liver, kid-by sulfonamides. They inhibit the conversion of neys and lungs; lower levels are achieved in mus-dihydrofolic acid to tetrahydrofolic acid by inhibit- cle and bone. Sulfonamides cross the placenta anding dihydrofolate reductase (DHFR). Bacterial and some may achieve therapeutic concentrations inprotozoal DHFR are significantly more tightly milk. Some are highly protein bound; protein bind-bound by the diaminopyrimidines than mam- ing varies with both the species and the drug. Themalian DHFR. When a diaminopyrimidine is com- Vd values in horses for sulfamethazine, sulfadox-bined with a sulfonamide, the action is synergistic ine and sulfadiazine are 0.63, 0.39 and 0.581/kg,and the combination is bactericidal. respectively. Diaminopyrimidines are weak bases. Peak plasma concentrations are reached early andMechanisms of resistance diaminopyrimidines are soon found in high con-Resistance to the sulfonamides occurs via chromo- centrations in tissues. In fact, the tissue concen-somal mutation or is plasmid mediated. Chromo- trations are often higher than the concentrations insomal mutation results in hyperproduction of serum. When inflammation is present, trimetho-PABA in bacteria, which overcomes the competi- prim levels in the CSF may reach 50% of thetive substitution of the sulfonamides. These muta- plasma concentrations. CSF concentrations oftions are of minor clinical significance. The most pyrimethamine are 25-50% of the plasma con-common form of bacterial resistance to sulfon- centrations. The Vd for trimethoprim andamides is via the plasmid-encoded production pyrimethamine is 1.51/kg in horses. The proteinof altered forms of DPS. More than 50 years of binding of trimethoprim is moderate (50%). ThereWidespread use of the sulfonamides in animal is no protein-binding interaction between thehealth has resulted in Widespread resistance. sulfonamides and the diaminopyrimidines.
  • 40. 2. ANTIMICROBIAL THERAPY 37 Sulfonamides are metabolized in the liver, usu- the sulfonamides; however, twice daily dosing isally by acetylation, glucuronidation and aromatic required to attain therapeutic plasma concentra-hydroxylation. The type of metabolite formed and tions of the diaminopyrimidines. the amount of each varies depending on the sul-fonamide administered and the species, age, diet Drug interactions and adverseand environment of the animal treated. The effects metabolites have little antimicrobial effect but maycompete with the parent drug in folic acid syn- Sulfonamides should not be administered with thesis. The kidneys eliminate parent drug and coumarin anticoagulants (e.g. warfarin) as their metabolites by glomerular filtration and active antimicrobial activity against vitamin Kesynthe- tubular secretion. Reabsorption, by passive diffu- sizing bacteria in the gastrointestinal tract cansion, occurs in the distal tubules. Most sulfon- increase the toxicity of the anticoagulant. Concur-amides are weak acids; therefore, they are rent use of bone marrow depressants may increasepredominantly ionized and excreted in alkaline the leukopenic and thrombocytopenic effects ofurine. In acidic urine they are unionized and folate antagonism of the sulfonamides or reabsorbed. In horses, only 43% of a dose of sul- diaminopyrimidines. It is assumed that animalsfamethazine is excreted in the urine and only have the same tendency as humans to develop7.8% of this is in the form of the unchanged par- signs of folate deficiency during long-terment drug. Relatively small amounts of the sulfon- administration or with high doses of folic acidamides are distributed into milk, saliva and the antagonists. Signs of folate deficiency includegastrointestinal tract. The half-lives in horses of agranulocytosis, megaloblastic anemia and throm-sulfamethazine, sulfadoxine, sulfadiazine and sul- bocytopenia. Complete blood and platelet countsfamethoxazole are 5.4-11,10-14,3-4 and 3.5-5h, should be performed on a regular basis, espe-respectively. cially during long-term or high-dose treatment. Trimethoprim is metabolized in the liver to Sulfonamides cross the placenta and are ter-oxide and hydroxyl metabolites. It is eliminated atogenic at very high doses. Potentiated sulfon-by glomerular filtration and active tubular secre- amides have been associated with abortion intion in the kidneys. In horses, a large percentage pregnant mares treated for equine protozoalof trimethoprim is metabolized before excretion myeloencephalitis (EPM; see Ch. 3) in the absencein urine (46%) and feces (52%). The clearance of of folate supplementation. However, folate admin-trimethoprim is affected by urine pH, plasma istration did not prevent fetal defects (renal andconcentrations and the degree of hydration. In bone marrow hypoplasia) in foals born to onehorses, the half-life of trimethoprim is 2-3 hand group of treated pregnant mares. Despite folatefor pyrimethamine it is 12h. supplementation, both mares and foals had lower There is considerable controversy regarding than normal serum folate levels. Folate and thedosage regimens of the potentiated sulfonamides, diaminopyrimidines use the same intestinal car-which are very difficult to determine based on rier for absorption, so if used concurrently theypharmacokinetic principles since each drug has a should be administered at least 2 h apart. The riskdifferent absorption, distribution and elimination. of congenital defects should be considered whenDifferent pathogens have different MICs and treating pregnant mares with potentiated sulfon-the optimal ratio of diaminopyrimidine to sul- amides. When trimethoprim/sulfamethoxazolefonamide varies depending on the pathogen. was given at recommended doses to pony stal-Clinically, the most important component of the lions, no changes in spermatogenesis were noted.formulation appears to be the diaminopyrimidine; However, these stallions developed neurologi-the choice of sulfonamide seems not to be nearly cal signs that could be mistaken for clinical signsas important. Registered veterinary potentiated of EPM.sulfonamide products are indicated for once daily Phenylbutazone may displace the sulfonamidesadministration based on the pharmacokinetics of from plasma protein-binding sites, increasing
  • 41. 38 EQUINE CLINICAL PHARMACOLOGYplasma sulfonamide concentrations. Procaine (in injections are painful. An injectable suspension ofprocaine benzylpenicillin (procaine penicillin)} is trimethoprim/sulfadoxine that is approved fora PABAanalog and may reduce the antimicrobial administration to cattle is often used in horsesactivity of the sulfonamides. Crystallization of (extra label). An injectable solution and oralthe sulfonamides may occur in the kidneys or in tablets and suspension containing trimethoprim/urine in dehydrated animals or following the sulfamethoxazole are approved for use in humansadministration of high doses. The solubility of and are used frequently in horses. Pyrimethaminethe sulfonamides in urine is dependent on urine is available as 25 mg tablets for p.o. administra-pH, urine drug concentration and the animals tion. Veterinary compounding pharmacies com-hydration status. The risk of crystalluria can be monly concoct a pyrimethamine/sulfadiazineminimized by maintaining a high urine flow and powder, which can easily be added to a horsesby alkalizing the urine. Hepatic insufficiency feed, for the treatment of EPM.may reduce sulfonamide metabolism and renalfunction impairment may reduce elimination,thereby increasing the risk of adverse effects. TETRACYCLINESRifampin increases the elimination rate oftrimethoprim and these agents should not be used Indicationsconcurrently. The potentiated sulfonamides have been asso- Tetracycline was discovered after a team of work-ciated with producing diarrhea in horses. In one ers examined 100000 soil samples from aroundstudy, changes in coliforms and clostridia were the world. Tetracycline derivatives include chlor-not seen following p.o. administration of tri- tetracycline, oxytetracycline, doxycycline andmethoprim/sulfadiazine. However, in a study of minocycline. The tetracyclines have a broad spec-hospitalized horses, the risk of diarrhea was sig- trum of activity; they are effective against Gram-nificantly increased when horses had been given positive and Gram-negative bacteria, somea potentiated sulfonamide and procaine benzyl- anaerobes, Chlamydia, Mycoplasma, Ehrlichia andpenicillin (procaine penicillin) concurrently. Rickettsia spp. and some protozoa. Their activity The injectable formulations of potentiated sul- against staphylococci is usually limited andfonamides are suspensions; consequently, rapid they are not active against enterococci. E. coli,i.v, administration causes hypotension and Klebsiella, Proteus and Pseudomonas spp. are usu-collapse. Fatal dysrhythmias are associated with ally resistant. Doxycycline and minocycline arethe potentiated sulfonamides administered i.v, usually more active in vitro than the other tetra-concurrently with the az-adrenergic agonist deto- cyclines. Differences in the clinical efficacy of themidine. It is suspected that the potentiated sul- tetracyclines can be attributed to differences infonamide formulation potentiates the cardiac the absorption, distribution and excretion of thechanges produced by detomidine. This adverse individual drugs rather than to differences inreaction has not been reported for the other bacterial susceptibility.az-adrenergic agonists xylazine and romifidine. Oxytetracycline and doxycycline are used in horses. Oxytetracycline is the drug of choice for Potomac horse fever, caused by Ehrlichia risticii,Formulations and equine ehrlichiosis, caused by Ehrlichia equi.Oral paste and powder formulations of trimetho- It is also used to treat contracted flexor tendons inprim/sulfadiazine are approved for use in foals, where the effects may be caused by calciumhorses. An injectable suspension of trimetho- chelation at the myotendinous junction resultingprim/sulfadiazine (48%) is available for i.v, or in the relaxation of the flexor tendons. Somei.m, administration to horses. Injections must be clinicians use parenteral oxytetracycline or p.o.administered slowly i.v, to avoid collapse and i.m. doxycycline to treat horses with EPM.
  • 42. 2. ANTIMICROBIAL THERAPY 39Mechanism of action and has the greatest degree of tissue penetration. The tetracyclines diffuse readily into milk. TheTetracyclines bind to the 30 S ribosomal subunit Vd of oxytetracycline is 0.34-0.951/kg in adultand interfere with bacterial protein synthesis. They horses and 2.17 11 kg in foals. The pharmacokinet-are bacteriostatic at normal therapeutic concentra- ics of doxycycline in horses has not been pub-tions, but bactericidal at high concentrations. They lished. Oxytetracycline is moderately proteinenter bacteria by passive diffusion and an active bound in horses (50%). Doxycycline is highlycarrier-mediated process. Mammalian cells do not protein bound in other species (90%).possess the tetracycline transport mechanism. The Tetracycline, chlortetracycline and oxytetracy-tetracyclines are most active at acidic pH, which is cline are excreted unchanged in urine, primarilyof benefit in the treatment of abscesses. by glomerular filtration. Parent drug is also elim- inated unchanged into the gastrointestinal tractMechanisms of resistance in bile. Doxycycline is excreted primarily in anAcquired bacterial resistance to the tetracyclines inactive form in the feces via non-biliary routes.has become Widespread in animal populations and Doxycycline does not accumulate in subjects withhas severely reduced the usefulness of these drugs. renal insufficiency. After i.v. administration toResistance to the tetracyclines results from plas- horses of oxytetracycline-PEG the half-life of drugmid-mediated mechanisms that prevent the active is 6h. Following i.m, administration, it has a "long-transport of the drug into the bacterial cell or acting effect" and a half-life of 22 h.increase the efflux of drug from the bacterial cell. Drug interactions and adversePharmacokinetics effectsThe tetracyclines are amphoteric compounds In horses, oxytetracycline administration has beenwith high lipid solubility. The absorption and associated with proliferation of Clostridium andbioavailability of oxytetracycline following par- Salmonella spp., resulting in potentially fatal coli-enteral administration varies depending on the tis. However, the cases reported typically involvedformulation and the injection site. Oxytetracy- stress, surgery, transportation and the concurrentcline formulated in polyethylene glycol (PEG) has use of multiple antimicrobial agents. Recent stud-a bioavailability of 83% following i.m. adminis- ies have demonstrated minimal changes in thetration to horses. Long-acting formulations, fecal flora and no other adverse effects in horsesapproved for use in cattle, have a rapid absorption given multiple i.m, doses of oxytetracycline-PEG.phase that is followed by a slow absorption phase Rapid i.v, administration of tetracyclines canto produce the "long-acting" effect. Absorption result in hypotension and collapse. This has beenfrom the neck muscles is typically more rapid and attributed to intravascular chelation of calciumcomplete than from the hindquarters. Tetracy- and/ or a decrease in blood pressure owing to theclines are often administered p.o. but their drug vehicle. The i.v, administration of doxycy-absorption can be erratic and unpredictable and cline to horses causes tachycardia, systemic arte-may be decreased by food. The bioavailability rial hypertension, collapse and death. This reactionfollowing p.o. administration is lowest for chlor- may be caused by the highly lipid-soluble doxy-tetracycline (30%), intermediate for tetracycline cycline chelating intracellular calcium, resultingand oxytetracycline (60-80%) and highest for in cardiac neuromuscular blockade.doxycycline (95%). Renal tubular necrosis following oxytetra- The tetracyclines are distributed into most tis- cycline administration is associated with highsues, except the eNS (therapeutic levels may be doses, expired parenteral products and concur-achieved when the meninges are inflamed). rent endotoxemia, dehydration and/or pigmentDoxycycline is the most lipid-soluble tetracycline nephropathy.
  • 43. 40 EQUINE CLINICAL PHARMACOLOGY Tetracyclines bind to teeth and bone. This may resistant to the cephalosporins and the aminogly-result in discoloration of the teeth and inhibition cosides. The MICs reported for the fluoro-of bone growth in fetuses exposed in utero. Tooth quinolones are very low and the MBCs are one todiscoloration may occur in young animals when two times the MIC for most pathogens. These aretetracyclines are administered during permanent the only p.o. antimicrobial agents available thattooth development. have efficacy against Pseudomonas spp. They are usually active against staphylococci, including MRSA. They have variable activity against strep-Formulations tococci and enterococci and are ineffective againstMany formulations containing oxytetracycline anaerobic bacteria. This may confer a therapeutic(50 or 100mg/ml) in propylene glycol (approved advantage in the treatment of enteric infections infor i.v, or i.m, administration to food-producing large animals, where anaerobes rarely cause dis-animals) are used extra label in horses. Long- ease and usually protect the gastrointestinal tractacting formulations of oxytetracycline (200mg/ml) by competitively inhibiting the colonization byformulated in PEG, 2-pyrrolidone, glycerol for- pathogenic aerobic organisms.mal or N,N-dimethylacetamide are approved foruse in cattle. Only the oxytetracycline in PEG for- Mechanism of actionmulation can be administered i.m, to horses; theother formulations are irritant. Oxytetracycline in The fluoroquinolones have a unique mechanismPEG or 2-pyrrolidone may be administered i.v, of action. They act directly on the bacterial cellbut as such will not be long acting. nucleus by inhibiting bacterial DNA gyrase (topoi- somerase). Bacteria have a single chromosome that consists of double-stranded DNA. Within the bacterial cell, the chromosome is folded around aFLUOROQUINOLONES RNA core and each DNA fold is supercoiled by DNA gyrase. DNA gyrase has four subunits: twoIndications A monomers and two B monomers. The enzymeThe quinolones are a group of synthetic antimi- forms a heart-shaped molecule, with the Acrobial agents. The first, nalidixic acid, was intro- monomers forming the "atria" and the Bduced in 1964. Nalidixic acid had good activity monomers the "ventricles". Bacterial DNA bindsagainst Gram-negative bacteria, but had a low to the topoisomerase in the cleft between the A andVd and produced serious side-effects, so its use Bsubunits. The enzyme nicks the double-strandedwas limited to treatment of urinary tract infec- DNA, introduces negative supercoils and resealstions. Further chemical manipulation resulted the DNA. The fluoroquinolones bind to thein the development of the fluorinated quinolones, DNA-DNA gyrase complex and inhibit thisagents with an extended spectrum of activity and resealing, resulting in an abnormal spatial config-improved safety.This group includes ciprofloxacin, uration of the DNA, which leads to its degrada-enrofloxacin, danofloxacin, difloxacin, sarafloxacin, tion by bacterial autolysins.norfloxacin, orbifloxacin, marbofloxacin, ofloxacin Fluoroquinolone activity is concentrationand ibafloxacin. New fluoroquinolones are dependent. All of the fluoroquinolones are bacte-being developed for use in human and veterinary ricidal; however, these drugs have an optimalmedicine. bactericidal concentration: higher or lower drug The fluoroquinolones have a broad spectrum concentrations result in reduced bactericidalof activity including most aerobic Gram-negative activity. It is thought that the DNA-DNA gyrasebacteria, some aerobic Gram-positive bacteria and complex has two binding sites for fluoro-Mycoplasma, Chlamydia and Rickettsia spp. They quinolones. At low drug concentrations, only oneare particularly effective against the enteric binding site is occupied, resulting in single-Gram-negative pathogens, including some strains stranded nicks remaining in the DNA. Reduced
  • 44. 2. ANTIMICROBIAL THERAPY 41killing at high concentrations is thought to be a but are not associated with resistance to otherconsequence of the dose-dependent inhibition of unrelated antimicrobial agents.RNA or protein synthesis. RNA and protein Fluoroquinolones must penetrate bacteria tosynthesis are required for the production of the reach their target, DNA gyrase. The second mecha-bacterial autolysins that are responsible for nism of fluoroquinolone resistance is decreasedquinolone-induced cell lysis. This explains the cell wall permeability. The fluoroquinolones dif-antagonism between fluoroquinolones and antimi- fuse through porin channels in the outer mem-crobial agents that inhibit RNA and protein syn- brane of Gram-negative bacteria. Mutation resultsthesis such as chloramphenicol and rifampin. in a decrease in porin channel proteins, resulting The fluoroquinolones are concentrated within in decreased uptake of the fluoroquinolones intophagocytic cells by simple diffusion. Intracellular bacterial cells. Alterations in a wide range of outerconcentrations may be several times greater than membrane proteins in Pseudomonas spp. result inplasma concentrations. Intracellular drug is micro- resistance. From these mutations, the increase inbiologically active; in vitro studies indicate that MIC of the fluoroquinolones is relatively low (2-ciprofloxacin reduces the survival of intracellular to 32-fold). However, there is cross-resistance withpathogens such as Brucella, Mycoplasma and unrelated antibiotics, most frequently cefoxitin,Mycobacterium spp. Exposure of Gram-negative chloramphenicol, trimethoprim and tetracycline.bacteria to fluoroquinolones at concentrations The third mechanism of resistance is increasedseveral times the MIC for 1-2 h results in a PAE of fluoroquinolone efflux, an energy-dependent1-6 h. This suggests that fluoroquinolone dosage process in the inner bacterial membrane thatregimens can include extended periods where exports drug into the periplasm or out of the cell.plasma concentrations are below the MIC of a This type of resistance has only been identified inpathogen. Staph. aureus and is associated with the presence of a resistance gene, norA. This appears to be a single point mutation, where alanine is substi-Resistance mechanisms tuted for asparagine. The degree of resistance Microbial resistance to the fluoroquinolones results conferred by this mechanism is less than that from chromosomal mutations that alter bacterial caused by gyrA mutation, but the increases in the DNA gyrase, decrease cell wall permeability or MICs are sufficient to produce resistant mutants increase fluoroquinolone efflux from bacterial cells. during fluoroquinolone therapy. In addition, thisThe most common mechanism of resistance is an mechanism of resistance has the potential toalteration in the bacterial DNA gyrase that results become plasmid mediated.in decreased fluoroquinolone binding to the The fluoroquinolones have been used inten-DNA-DNA gyrase complex. This occurs as a result sively in human medicine in the 1990s, resultingof point mutations in the gene coding for the A in a high level of resistance in some pathogens. Insubunit of the enzyme. Most mutations identified resistant strains, more than one mechanism isinvolve substitution of the serine at position 83 responsible for resistance; usually two or threeby leucine or tryptophan. The substitution of the mechanisms operating in conjunction. In resist-hydroxyl group of the serine residue with the ant Staph. aureus, increased efflux is often cou-bulkier hydrophilic groups of leucine and trypto- pled with gyrA mutation. In resistant E. coli, gyrAphan results in physical obstruction of the binding mutation is usually associated with changes inof the fluoroquinolone to the DNA-DNA gyrase the outer membrane proteins.complex. The quinolone-sensitive A subunit (As)is dominant over the quinolone-resistant A sub-unit (A): DNA gyrase composed of both As and PharmacokineticsA r subunits is as sensitive as gyrase composed of Injections of enrofloxacin i.v, are well toleratedAs subunits alone. Gyrase mutations confer a high by horses. Enrofloxacin is well absorbed afterlevel of cross-resistance to all fluoroquinolones i.m. administration but is irritant. The
  • 45. 42 EQUINE CLINICAL PHARMACOLOGYfluoroquinolones are rapidly and well absorbed antagonistic to chloramphenicol or rifampin owingfrom the gastrointestinal tract of monogastric ani- to the inhibition of bacterial autolysin synthesismals and preruminant calves. The bioavailability following the concurrent administration ofof enrofloxacin is 50-60% after p.o. administra- bacterial protein synthesis inhibitors. The fluoro-tion to horses, but for ciprofloxacin in ponies it is quinolones interfere with the metabolism ofonly 6%. methylxanthines such as theophylline. Serum The fluoroquinolones are extremely lipid solu- concentrations of theophylline or aminophyllineble and distribute well. The Vd of enrofloxacin is may double and so must be monitored during2-51/kg in most species. Tissue concentrations concurrent therapy.greatly exceed plasma concentrations during ther- Chronic administration of high doses of theapy. Extremely high concentrations are achieved fluoroquinolones causes articular cartilage lesionsin the kidneys, urine, liver and bile. After multiple in young puppies. Transient arthropathy occursp.o. doses, urine, endometrial tissue and synovial when ciprofloxacin is used in the therapy offluid concentrations in horses are higher than Pseudomonas spp. pneumonia in children withserum concentrations. Therapeutic concentrations cystic fibrosis. Arthropathy has not been reportedmay be achieved in the CSF. High concentrations in calves, swine, or poultry. Arthropathy has beenoccur in milk. The protein binding is low (22%). documented in 2-week-old foals given enrofloxacin The fluoroquinolones are predominantly p.o. at a dose rate of 10 mg/kg but was not seenexcreted unchanged in urine following glomerular in adult horses given enrofloxacin i.v. at up tofiltration and active tubular secretion. Enrofloxacin 25mg/kg for 3 weeks. Despite the risk of arthro-is partially metabolized (de-ethylated) to cipro- pathy, the fluoroquinolones have been used withfloxacinand other active metabolites. Ciprofloxacin clinical success in septic foals when the treatmentconcentrations reach 20-35% of enrofloxacin options were limited. Joint lesions are most severeconcentrations in serum. For some pathogens, in weight-bearing, diarthrodial joints, so if fluoro-the MICs for ciprofloxacin are lower than for quinolones are administered to foalsexerciseshouldenrofloxacin; therefore, ciprofloxacin contributes be severely limited.to the efficacy of enrofloxacin. The half-life of In general, the fluoroquinolones are not rec-enrofloxacin in horses is 6-7h, while for ommended for use during pregnancy. However,ciprofloxacin in ponies it is 2.5h. the fluoroquinolones available currently are not teratogenic in laboratory animals. Moreover, women treated with ciprofloxacin and pregnantDrug interactions and adverse mares treated with enrofloxacin did not have aneffects increase in abortions or birth defects.The fluoroquinolones have been used in combi- Hallucinations, photosensitivity reactions andnation with other antimicrobial agents to expand Achilles tendon rupture have been associatedtheir therapeutic spectrum, to suppress the emer- with fluoroquinolone use in humans but havegence of drug-resistant bacterial populations or not been reported in animals.to exploit inhibitory or bactericidal synergism (afour-fold or greater decrease in MIC or MBC for Formulationseach antimicrobial) against drug-resistant popu-lations. There is minimal synergy between the Ciprofloxacin (40mg/ml) is approved for i.v,fluoroquinolones and the beta-lactams or amino- administration to humans. This formulationglycosides against Gram-negative enteric bacteria. must be diluted and administered slowly and isThis may be because of the already high suscepti- extremely expensive. Enrofloxacin is available asbility of these organisms to the fluoroquinolones. an injectable solution (50mg/ml) and tablets forCombination with the beta-lactams, aminoglyco- p.o. administration to small animals and as asides or vancomycin is additive or indifferent solution (100mg/ml) for s.c. administration toagainst staphylococci. The fluoroquinolones are cattle. Both of these injectable solutions may be
  • 46. 2. ANTIMICROBIAL THERAPY 43administered safely by i.v, injection (extra label) Pharmacokineticsto horses and the tablets for small animal may be Erythromycin formulations are highly irritant ifground up and administered p.o. to horses. administered by i.m. injection and are not used in horses. Many p.o. preparations of erythromycin are enteric coated to allow passage into the smallMACROLIDES intestine, where absorption is higher because of the higher pH. In horses, erythromycin stearateIndications and erythromycin phosphate produce peakThe macrolide antibiotics include erythromycin, plasma concentrations faster than the esterclarithromycin, azithromycin, tylosin, tilmicosin formulations following p.o. administration.and tiamulin. Clindamycin and lincomycin are The macrolides distribute well and tissue con-related lincosamides. Susceptible bacteria include centrations may be higher than serum concentra-staphylococci, streptococci, Campylobacter jejunii, tions. Erythromycin concentrates and is active inClostridium spp., R. equi, Mycoplasma pneumoniae leukocytes because of its high lipid solubility andand Chlamydia spp. Drugs in this group are only ion trapping. The Vd of erythromycin is 3.7-effective against a few Gram-negative bacteria in 7.211 kg in adult horses and foals. The protein bind-cattle, namely some strains of Pasteurella and ing is low. The hepatic clearance of the macrolidesHaemophilus spp. Macrolides and lincosamides may be slower in animals of up to 1 month of ageare associated with causing colitis in horses, so than in adult animals.their use is usually restricted to p.o. erythromy- Erythromycin is extensively metabolized, withcin for the treatment of R. equi infections in foals. much of the unchanged parent drug and activeSubantimicrobial doses of erythromycin are metabolite eliminated in bile, resulting in a shortadministered i.v, to horses for gastrointestinal half-life (1-3h). Erythromycin undergoes entero-prokinetic action. hepatic recirculation, which may contribute to the adverse gastrointestinal effects seen in adult horses.Mechanism of actionMacrolides bind to the 50S ribosomal subunit, in Drug interactions and adversea manner similar to chloramphenicol and flor- effectsfenicol, and interfere with bacterial protein syn- Macrolides are metabolized in the liver via thethesis. They are usually considered bacteriostatic microsomal (cytochrome P450) enzyme system.but may be bactericidal at high concentrations. The alkylxanthines (e.g. theophylline, amino-The antimicrobial activity of erythromycin is pH phylline) utilize the same enzyme system, so con-dependent. Optimum activity occurs at a pH of current administration with macrolides leads to a8.8; activity is reduced in acidic environments doubling of the alkylxanthine concentration andsuch as abscesses. toxicity. Because of similar mechanisms of action, concurrent administration of other macrolides, lincosamides, chloramphenicol or florfenicol isMechanisms of resistance not recommended.The routine use of the macrolides is limited Agents in this group, especially clindamycinbecause bacterial resistance develops quickly and lincomycin, are associated with bacterial over-following repeated exposure. There can be cross- growth in the colon. Serious and potentially fatalresistance between the drugs in this class. The diarrhea may occur in humans, rabbits, rumi-mechanisms of resistance include decreased drug nants and horses. Foals appear to be less suscep-entry into bacteria, inability to bind to the 50S tible to erythromycin-induced diarrhea thanribosomal subunit and the plasmid-mediated adult horses and the ethylsuccinate formulationproduction of macrolide-destroying esterase. seems to be the least likely to induce diarrhea.
  • 47. 44 EQUINE CLINICAL PHARMACOLOGYFatal colitis has been reported in mares with foals susceptible organisms. It has no effect on thethat were being treated with p.o. erythromycin. mammalian enzyme. Its action is bacteriostatic orErythromycin may also induce diarrhea because bactericidal depending on the susceptibility ofof its prokinetic action. It mimics the effects of the the organism and the concentration of the drug.hormone motilin on the enteric nervous system,and subantimicrobial doses stimulate small intes- Mechanisms of resistancetinal, cecal and colonic motility. Administration of erythromycin i.m. is very Chromosomal mutation develops readily in mostpainful and this route is not used in horses. The bacteria exposed to rifampin and leads to a highi.m. formulation cannot be safely administered i.v level of resistance. These mutants show stable Erythromycin may reduce the normal ther- changes in RNA polymerase that prevent bind-moregulatory response to hyperthermia. Erythro- ing. Resistance to rifampin is not transferable andmycin has been associated with hyperthermia in there is no cross-resistance with other antibiotics.foals. Treated foals that are turned out on hot,sunny, humid days develop fever, tachypnea and Pharmacokineticsdistress, which may result in fatal heat stroke. The bioavailability of rifampin following p.o.Formulations administration to horses is 40-70%. The bioavail- ability is reduced if given with feed, but the cur-Erythromycin gluceptate and erythromycin lacto- rent dosage regimens compensate for this.bionate are approved for i.v, use in humans. Rifampin is very lipophilic and penetrates mostSeveral delayed-release p.o. formulations are avail- tissues including mammary glands (milk), bone,able because erythromycin is unstable in gastric abscesses and the CNS. It is concentrated in neu-acid: enteric-coated erythromycin tablets and trophils and macrophages and is effective againsterythromycin stearate, ethylsuccinate and estolate. intracellular pathogens. It is most active at acid pH, making it a rational choice for the treatment of septic foci and granulomatous infections. InRIFAMPIN horses, the V d of rifampin is 0.91/kg. Rifampin is highly protein bound.Indications Rifampin is metabolized in the liver to aRifampin is effective against Staph. aureus, deacetylated metabolite that also has antibacterialHaemophilus spp., R. equi and a variety of activity. Unchanged drug and the active metabo-mycobacteria. At very high concentrations, it has lite are eliminated primarily in bile, but up to 30%activity against poxviruses and adenoviruses. may be excreted in urine. The parent drug under-Rifampin also has antifungal activity when com- goes extensive enterohepatic recirculation, but thebined with other antifungal agents. Resistance metabolite does not. The half-life of rifampin isdevelops rapidly; therefore, it is usually adminis- 6-8h in adult horses and 17h in foals. Repeatedtered concurrently with another antimicrobial dosing results in a decrease in the half-life ofagent. In equine practice, is most commonly used rifampin through induction of hepatic enzymes;in combination with erythromycin for the treat- this may also affect the metabolism of concur-ment of R. equi infections in foals. It may also be rently administered drugs.used in the treatment of refractory osteomyelitisand endocarditis caused by Staph. aureus. Drug interactions and adverse effectsMechanism of action Microsomal enzyme induction may shortenRifampin suppresses RNA synthesis by selectively the half-life of rifampin and decrease druginhibiting DNA-dependent RNA polymerase in concentrations of concurrently administered
  • 48. 2. ANTIMICROBIAL THERAPY 45chloramphenicol, corticosteroids, itraconazole, of 58-91 %. Therapeutic concentrations are attainedketoconazole, warfarin and barbiturates. Rifampin after administration of metronidazole per rectumstains everything it contacts red and treated horses to horses with gastrointestinal stasis.will also produce red urine, feces, tears, sweat Metronidazole is lipophilic and distributesand saliva. This has no harmful consequences! widely. Peritoneal fluid and milk concentrations approach plasma concentrations. It reaches thera- peutic concentrations in bone, abscesses and theFormulations CNS. It readily crosses the placenta and enters theCapsules for p.o. administration are approved for fetal circulation. In most species, the V d is 1-2I1kg.use in humans. Metronidazole is metabolized primarily in the liver. Both unchanged drug and metabolites are eliminated in urine and feces. The half-life of metronidazole in horses is 3-4 h.METRONIDAZOLEIndications Drug interactions and adverseMetronidazole is a unique antimicrobial agent that effectshas very little effect on most aerobic Gram-positive Metronidazole is mutagenic (in bacteria) and car-and Gram-negative bacteria but is highly effective cinogenic (following prolonged treatment of lab-against anaerobic bacteria, including B. fragilis oratory mice); therefore, its use is prohibited in(penicillin-resistant) and Fusobacterium and food-producing animals. It has also been impli-Clostridium spp. It also has good activity against cated as being teratogenic in laboratory animalsprotozoa (e.g. Giardia and Trichomonas spp.). In and so should be avoided in pregnant animals.horses, it is used to treat sequestered anaerobic Neurotoxicity may occur with metronidazoleinfections or infections caused by bacteria resist- at recommended doses during prolonged use orant to the beta-lactam antibiotics. at high doses. The signs of this include ataxia, lethargy, anorexia, nystagmus and seizures.Mechanism of action Affected animals may recover with supportive therapy.Bacteria rapidly take up metronidazole and reduceit to cytotoxic short-lived free radicals. These com-pounds damage DNA and other critical intracel- Formulationslular macromolecules. Aerobic bacteria lack the Metronidazole is available as tablets and capsulesreductive pathway necessary to produce these free for human use.radicals.Mechanisms of resistance VANCOMYCINResistance involves reduced intracellular drug Indicationsactivation and is rare among usually susceptiblebacteria. There is one report of metronidazole- Vancomycin is a glycopeptide antibiotic that isresistant isolates of B. fragilis from a horse with very important in human medicine because of itspleuropneumonia. activity against multidrug-resistant organisms such as MRSAand enterococci. Vancomycin is only active against Gram-positive bacteria. In recentPharmacokinetics years, nosocomial infections of vancomycin-Metronidazole is rapidly and well absorbed after resistant enterococci (VRE) have become a majorp.o. administration to horses, with a bioavailability problem in human hospitals. Recently, a MRSA
  • 49. 48 EQUINE CLINICAL PHARMACOLOGYresistant to vancomycin was isolated from human Martinez M N 1998a Physicochemical properties of pharmaceuticals. Journal of the American Veterinaryclinical patients, causing a great deal of concern in Medical Association 213:1274-1277human medicine because of the lack of treatment Martinez M N 1998b Volume, clearance and half-life. Journaloptions. Vancomycin inhibits synthesis of bacter- of the American Veterinary Medical Association 213:1122-1127ial cell wall phospholipids and polymerization of Martinez M N 1998c Clinical application ofpeptidoglycan. Vancomycin resistance results from pharmacokinetics. Journal of the American Veterinaryplasmid-mediated changes in cell wall perme- Medical Association 213:1418-1420 Prescott J F, Holgate S T 1993 Antimicrobial susceptibilityability and decreased binding of vancomycin to and drug dosage prediction. In: Prescott J F, Baggot J Dreceptor molecules. Vancomycin is expensive, (eds) Antimicrobial therapy in veterinary medicine, 2ndmust be given parenterally and causes serious edn. Iowa State University Press, Ames, lA, pp. 11-20 Riviere J E 1999 Comparative pharmacokinetics: principles,adverse effects. There are only a few reports of the techniques and applications. Iowa State Universityuse of vancomycin in veterinary medicine. Press. Ames, lA, pp. 47-61 Riviere J E, Webb A I, Craigmill A L 1998 Primer on estimating withdrawal times after extra label drug use.REFERENCES Journal of the American Veterinary Medical Association 213:966-968Baggot J D 1992 Bioavailability and bioequivalence of Verbist L 1993 Relevance of antibiotic susceptibility testing veterinary drug dosage forms, with particular reference for clinical practice. European Journal of Clinical to horses: an overview. Journal of Veterinary Microbiology and Infectious Diseases 12(suppl 1):2-5 Pharmacology and Therapeutics 15: 16D-173 Vogel man B, Gudmundsson S, Leggett J et al 1988Freeman D E 1999 Gastrointestinal pharmacology. Correlation of antimicrobial pharmacokinetic parameters Veterinary Clinics of North America Equine Practice with therapeutic efficacy in an animal model. Journal of 15:535-559 Infectious Diseases 158:831-847 Appendix: Overview of the pharmacokinetics, Indications and dose rates of antimicrobial agents used In the horse Antimicrobial Volume of Half-life Acld-base Protein Elimination Indications Dose agent distribution (h) status binding (1/kg) (%) Potassium penicillin, 0.2-D.3 Acid 52-54 RE Streptococcal 200001U/kg, sodium penicillin infections; anaerobic q.l.d, infections Procaine 7 Acid RE Streptococcal 25000lU/kg, benzylpenicillin infections; anaerobic b.l.d. (procaine penicillin) infections Ampicillin 0.18 0.5-1.5 Acid 6.8-8 RE Gram-positive, some 1D-20mglkg Gram-negative and Lv. or l.rn.. anaerobic infections t.l.d, to q.l.d. Cefazolin 0.19 0.6-D.8 Acid 8 HM, RE Gram-positive, some 15mg/kg t.v., Gram-negative and b.l.d, to t.i.d. anaerobic infections Cefalotin 0.15 0.25 Acid 18 HM, RE Gram-positive, some 2Q-40mg/kg Gram-negative and i.v or i.rn., anaerobic infections t.i.d. to q.l.d, Cefapirin 0.17 0.9 Acid RE Gram-positive, some 30mglkg l.v, Gram-negative and or l.rn, every anaerobic infections 4-6h Cefoxitin 0.12 0.8 Acid RE Gram-positive, some 20mg/kg i.v, Gram-negative and every 4-6h anaerobic Infections (continued)
  • 50. 2. ANTIMICROBIAL THERAPY 47(continued)Antimicrobial Volume of Half-life Acid-base Protein Elimination Indications Doseagent distribution (h) status binding (I/kg) (%)Ceftiofur 3-5 Acid 99 HM. RE Gram-positive. some 2.2 mg/kg Gram-negative and l.m. once anaerobic infections dailyGentamicin 0.12-0.24 1-2 Base <30 RE Staphylococcal and 5-7 mg/kg (adult). (adult) Gram-negative i.v.• i.rn.• s.c. 0.3 (foal) infections once dailyAmikacin 0.14-0.22 1-3 Base RE Infections with 6mglkg i.v.. (adult). (adult), bacteria resistant to i.m .. s.c. 0.4-0.6 5 (foal) gentamicin once daily; (foal) 20mg/kg i.v. once daily in neonatal foalsChloramphenicol 1.4 (adult). <1 Base Low HM. RE Gram-positive. some 45-60mg/kg 1.6 (foal) Gram-negative and l.v.. i.rn., s.c. anaerobic infections or p.o .. t.i.d. to q.i.d.Florfenicol 0.72 1.8 Base Low HM. RE Cautious use with 20mg/kg i.rn, infections from every 4-8h resistant bacteriaSulfonamides 0.3-0.6 Variable Acid Varies HM. RE Gram-positive, some 25mg p.o. Gram-negative and b.i.d. protozoal infectionsTrimethoprim 1.5 2-3 Base 50 HM. RE Gram-positive. some 5mg p.o. Gram-negative and b.l.d, anaerobic infectionsPyrimethamine 1.5 12 Base 70-80 HM. RE Protozoal infections 1 mg/kg p.o. once dailyOxytetracycline 0.34-0.95 6 (i.v.), Amphoteric 50 HM. RE Gram-positive. some 7-10mg/kg (adult). 22 (l.rn.. Gram-negative and Lv. once daily; 2.2-4 long acting) anaerobic infections 20mg/kg i.m. (foal) every 72 h (long acting)Enrofloxacin 2-5 6-7 Amphoteric 22 HM, RE Staphylococcal and 5-10mglkg Gram-negative i.v or p.o. infections once dailyErythromycin 3.7-7.2 1-3 Base Low HM. RE R. equi infections 25 mg/kg p.o. once dailyRifampin 0.9 6-8 Amphoteric 89 HM, RE R. equi infections 5-10mg/kg (adults), p.o. once 17 (foals) dailyMetronidazole 1-2 3-4 Base HM. RE Anaerobic infections 15-25mg/kg q.t.dHM. hepatic metabolism; RE. renal elimination; IU, international units; q.l.d.. fourtimesdaily; b.i.d .• twicedaily; i.v.• intravenous; l.m..intramuscular; l.i.d.. three timesdaily; s.c.. subcutaneous
  • 51. CHAPTERCONTENTSIntroduction 49Piroplasmosis/babesiosis 49 Parasite biology 49 Antiprotozoal drugs Pathogenesis, clinical signs and diagnosis 50 Treatment 51 Clara K. FengerTrypanosomiasis 53 Parasite biology 53 Pathogenesis, clinical signs and diagnosis 53 Treatment 55Giardiasis 57 Parasite biology 57 Treatment 57Coccidiosis (globidiosis) 57 Parasite biology 57 Pathogenesis, clinical signs and diagnosis 57 INTRODUCTION Treatment 57Sarcocystis infections: equine protozoal Horses act as the natural and aberrant hosts for amyeloencephalitis 58 number of protozoan species spanning a wide Parasite biology 58 range of phyla. This wide diversity means that the Pathogenesis, clinical signs and diagnosis 58 Treatment 59 drug classes used for the treatment of these differ- ent protozoa may be effective against one speciesOther Sarcocystis infections 61 and not others. Thus, this chapter is organized by Parasite biology 61 Pathogenesis, clinical signs and diagnosis 62 pathogen rather than by drug class. Treatment 62References 62 PIROPLASMOSIS/BABESIOSIS Protista Apicomplexa Pyroplasma Pyroplasmida Babesiidae Babesia. PARASITE BIOLOGY Species of the genus Babesia are intracellular ery- throcyte pathogens transmitted by ticks, with mechanical transmission possible by biting flies such as Tabanus spp. Equine piroplasmosis is caused by B. caballi and B. equi, both of which utilize horses, mules and donkeys as hosts. B. caballi is dis- tributed throughout temperate Eurasia (transmit- ted by Dermacenter reticulatus and D. marginatus), the Mediterranean basin and central-western Asia (transmitted by Hyalomma m. marginatum), North Africa (transmitted by Hyalomma spp.) and tropi- coequatorial America (transmitted by Dermacenter nitens). The known natural tick vectors for B. equi are the tropical horse tick, D. nitens in the western hemisphere, Rhipicephalus bursa in the Mediter- ranean basin and Rhipicephalus evertsi in tropico- equatorial Africa. Dermacenter and Hyalomma 49
  • 52. 50 EQUINE CLINICAL PHARMACOLOGYspp. may be implicated in other regions of Africa asexual reproduction or schizogony. The mero-and Boophilus microplus has been shown to be zoites of B. equi have a developmental stage incapable of supporting complete development of lymphocytes prior to entry into erythrocytes.B. equi experimentally. As the protozoa reproduce, the erythrocytes are The B. caballi vectors are ticks with two and destroyed releasing the piroplasms into the blood-three hosts and only the adult ticks feed on horses stream and clinical signs of hemolytic anemiaand transmit this protozoan. The adult female tick occur. The severity of these clinical signs is corre-ingests the B. caballi merozoite-infected erythrocyte lated to the degree of parasitemia in both formsduring its final blood meal before egg laying. The of piroplasmosis.merozoites develop into macrogametocytes, which B. caballi produces a great variety of clinicalmay continue development as a macrogametocyte signs and variable disease progression: it may be(female) or develop into microgametocytes (male) acute or chronic and mild or severe. The incubationby exflagellation. After fertilization, the zygote period is 7-19 days; the level of parasitemia isgoes on to develop into a sporoblast in the cells of high (50-100%) and there is 10-20% mortality.the digestive epithelium of the tick. The blasto- Anemia and icterus without hemoglobinuria arezoites develop within these cells, elongate and typical. Locomotor disorders and pelvic limbthen escape into the hemocoel of the tick. These paralysis, related to cortical congestion, are seenblastokinetes (motile protozoa) penetrate multiple in the acute forms of this disease. Lymphadenitis,tissues of the tick including the ovaries, resulting peritonitis and pericarditis are usually evident.in transovarial transmission. During the embryo- Horses with chronic disease may have poor gen-genesis of the tick, the protozoans multiply, going eral condition, colic, lethargy and inappetance.through several stages of blastozoite, sporoblast Although the mortality rate may be as high as 20%and then blastokinete. The terminal blastozoites in naive populations of horses, it is much lower inin the salivary glands of the juvenile tick develop endemic regions. If untreated, horses that recoverinto trophoblasts, which develop into terminal remain persistently infected. In general, horsessporocysts; these produce the infective form of that have recovered have a subclinical infectionthe parasite, the metacyclic sporozoites. and act as a reservoir of infection for other horses. The vector ticks of B. equi are infected at the The disease is considered exotic in the USA,nymph stage by feeding on an infected equine Canada, Australia, New Zealand and Japan, wherehost and the protozoan is passed transstadially, control measures are in place to prevent its intro-through the development stages of the tick to the duction. These controls limit the movement ofadult tick, where the infection can be transmitted equines and have a profound effect on the equineto another susceptible equine host. B. equi differs industry in endemic regions.from B. caballi and other Babesia spp. in terms B. equi usually causes subacute or acute dis-of biological behavior, molecular phylogenetics ease after an incubation period of 10-21 days.and chemosensitivity, suggesting that it may Moderate (10-30%) and transient (3-5 days)belong to a different genus. Some authors have, parasitemia occur and there is 20-50% mortality.in fact, reclassified this organism as Theileria equi If the disease is peracute, death may occur within(Melhorn & Schein 1998). However, it will be con- 2 days of the onset of infection. In acute cases, thesidered here in the section on piroplasmosis and process lasts for 8-10 days, after which the ani-the differences in chemosensitivity discussed. mal recovers and becomes a carrier. The first clin- ical sign, around 10 days after a tick bite, is typically high fever (up to 41°C/105.8°F) accom-PATHOGENESIS, CLINICAL SIGNS panied by anemia, hemoglobinuria, icterus andAND DIAGNOSIS a rapid respiratory rate. Lethargy, depression,Infective metacyclic sporozoites in tick saliva are anorexia and edema of the head, ventral abdomenpassed directly into the host, where they immedi- and limbs ensue as the disease progresses. Ascitesately enter erythrocytes and begin to undergo and petechial hemorrhages may be evident.
  • 53. 3. ANTIPROTOZOAL DRUGS 51 Equine piroplasmosis is usually diagnosed by Diminazene aceturateexamining (peripheral) blood smears for the par- Diminazene aceturate is an odorless, yellow pow-asites. This is most sensitive during the acute phase der that is soluble in water up to a concentrationof the infection because the parasites are not usu- of 7%. It is active, stable, has low toxicity and hasally visible during the latent phase. In the ~atent bactericidal, babesicidal and trypanocidal prop-phase and in subclinical d.isease, serol?gy IS the erties; it appears to bind directly to parasites. It ismost effective method of disease detection, There effective in the treatment of babesiosis in cattle,are complement fixation tests specific for B. caballi sheep and dogs: intramuscular (i.m.) adm~istra­and B. equi, respectively, although there may be tion of diminazene aceturate at 3.5 mg/kg IS fol-some cross-reactivity between the two tests. Both lowed by the disappearance of the clinical signsfalse-positive and false-negative test results can of babesiosis within 24 h. In horses, the recom-occur, confounding any attempt to view the results mended regimen is two doses administeredof these tests as absolute. intramuscularly 24h apart at Smg/kg for the treatment of B. caballi and at 6-12mg/kg for B.TREATMENT equi. At these dose rates, the drug effectively clears B. caballi infection and eliminates theB. caballi can be treated effectively but there is not clinical signs but does not clear B. equi infection.a comparable chemotherapeutic approach to the It is effective at similar dose rates againsttreatment of B. equi, where drugs may control the Trypanosoma congolense and T. vivax infections butclinical signs of infection but horses often remain less active against T. brucei, which requires a doselife-long carriers. In regions where piroplasmosis of 7mg/kg. In persistent trypanosome infections,is endemic, elimination of the infection may not dose rates of up to 8 mg/kg should be given,be desirable because these horses will then be sus- divided into two doses administered 4 h apartceptible to reinfection. In these areas, premuni- and split between two or three different injectiontion is used: the horses are infected or allowed to sites.become infected and are then treated with suffi- In horses, diminazene aceturate should becient chemotherapy to control the clinical signs administered by deep i.m. injection dividedbut not to eliminate the infection. Horses that between several injection sites. The injection siteshave been premunised are thus infected but not should be massaged in order to promote drugaffected. This technique permits the development absorption. At the dose rates suggested here, ~ignsof the carrier state and as a result some resistance of toxicity are uncommon but local reactions,(immunity) to reinfection. caused by muscle necrosis, may be severe. Dimi- nazene aceturate should be avoided in horsesAromatic diamidines unless other drugs are either ineffective or unavailable. Toxic doses result in respiratory dis-The mechanism of action of this group of com- tress, depression, cardiac signs, hypersalivationpounds is not known but is related to their guanyl and diarrhea. Toxic doses may be treated withgroup NH=(NH 2) . Ultrastructural studies show calcium salts.that these compounds cause dilatation of mem-brane-bound organelles, dissolution of the cyto-plasm and destruction of the nucleus in Babesia Phenamidine isoethionatespp. Compounds with two guanyl groups sepa-rated by a hydrocarbon chain have a high level of Phenamidine isoethionate is a bitter, white, odor-babesicidal activity, which increases with the length less, water-soluble compound. In horses, it is usedof the hydrocarbon chain. There are three com- at 8-13mg/kg i.m, or subcutaneous (s.c.); it ispounds in this class that are used in the treatment limited to the treatment of B. caballi infectionsof piroplasmosis in horses: diminazene aceturate, because it is ineffective in the treatment of B. equiphenamidine isoethionate and pentamidine. infections and trypanosomiasis.
  • 54. 52 EQUINE CLINICAL PHARMACOLOGYPentamidine concentrations of muscle enzymes and mild hepatic peripheral lobular necrosis (Taylor et alPentamidine is rarely used in horses but may beeffective against B. caballi, T. brucei and T. evansi. 1972). Amicarbalide is toxic at very high dosesIt must be administered by slow intravenous and the toxicity includes hepatic and renal tubular necrosis.(i.v.) injection or i.rn, injection using multiplesmall injection sites and the treatment repeatedafter 48h. Imidocarb dipropionate Imidocarb is an off-white, water-soluble powder.Complex urea compounds This compound distributes well and can be foundThe complex urea compounds are related to the in tissues up to 4 weeks after a single i.m. injection.aromatic diamidines. Their mechanism of action Imidocarb is mainly eliminated unchanged inis unknown. urine; about 10% is excreted in feces. A single i.m. injection of imidocarb diproprionate 2.2mg/kg can be used to ameliorate the clinical signs of B.Quinuronium sulfate cabal/i, with two doses administered 24h apartQuinuronium sulfate is a bitter, white to yellow, required to clear this infection. However, fourcrystalline powder that is usually available as a doses at 4 mg/kg administered 72h apart are notstable 5% aqueous solution. This compound is consistently effective in eliminating B. equi infec-effective in the treatment of B. caballi infections tion. Imidocarb dipropionate is well tolerated atbut is associated with relapses, making it more i.m. injection sites. Horses may exhibit markedeffective for premunition than for the elimination transient side-effects, including extreme restless-of infection. One treatment consists of two doses of ness, sweating, colic and persistent anorexia fol-a 5% solution of quinuronium sulfate, adminis- lowing imidocarb diproprionate administration.tered s.c. at O.3mg/kg, 6h apart. Quinuronium These side-effects can be ameliorated or avoidedsulfate has a narrow margin of safety and over- by pretreatment with atropine sulfate (1% solutiondosing produces parasympathomimetic effects 1 ml/100kg) and by dividing the imidocarb doseincluding tremors, salivation, urination and defe- into two injections given 3 h apart.cation. These signs usually respond to treatmentwith atropine, epinephrine (adrenaline) and cal-cium gluconate. The interval between treatments Tetracyclines (oxytetracycline)should not be shorter than 2 weeks and should The tetracyclines are amphoteric antimicrobialpreferably be 3 months because sensitization agents that can form salts with bases or acids (seeoccurs, which results in shock, with a profound Chs 1 and 2). Oxytetracycline is a bitter, yellow,drop in blood pressure, and death. odorless crystalline powder. The base is slightly water soluble and the hydrochloride is readily water soluble and is typically administered toAmicarbalide horses by slow i.v, injection. It is effective atLow doses of amicarbalide can be used for pre- 5.5 mg/kg once daily for 2 days or more inmunition; however, relapses are frequent. Two the treatment of B. equi but is unlikely to com-doses administered i.m. at 8.8mg/kg 24h apart pletely clear this infection. It is, therefore, usedare effective in eliminating B. caballi infections. for premunition. Rapid i.v, injection may cause aHowever, 22 mg/kg administered i.m. once daily precipitous drop in blood pressure and collapsefor 7 days is only 50% effective in eliminating owing to the effects of calcium chelation on theB. equi infections. At therapeutic doses, there is myocardium. Intramuscular injection causesswelling, inflammation and necrosis at the i.m, objectionable local reactions in horses and shouldinjection site, resulting in increased serum be avoided. Oral administration may be more
  • 55. 3. ANTIPROTOZOAL DRUGS 63likely to be associated with colitis in horses than PARASITE BIOLOGYi.v, administration, although colitis may occurfollowing administration by any route. Trypanosomes are flagellate hemoprotozoans that live extracellularly in the blood. Most Trypanosoma spp. are transmitted by tsetse flies (Glossina spp.)Other compounds in endemic regions of tropicoequatorial AfricaTrypan blue (Trypan red) but they may also be transmitted by mechanical vectors, including horse flies (Tabanus spp.), orThe Trypan dyes are bisazo compounds distantly by the venereal route, in the case of T. equiperdumrelated to the sulfonamides. Trypan blue, a bluish- (dourine). Mechanical transmission is the pri-gray, water-soluble powder, was one of the first mary source of infection in non-tsetse regionsdrugs to be used for the treatment of piroplasmo- including parts of Africa, Central and Southsis and trypanosomiasis. Trypan blue is effective America, West India and Mauritania.against B. caballi but not B. equi. In horses, a Trypanosomes are ingested by the tsetse flyfreshly prepared 1-2% solution of Trypan blue can during a blood meal and enter the midgut, wherebe administered by slow i.v. injection at 2-3 mg/kg they transform into procyclic trypomastigotes thatfor premunition. Rapid i.v, administration of are capable of multiplication. These migrate to theTrypan blue may result in shock. Subcutaneous flys salivary glands and go through the epi-administration of Trypan blue should be avoided mastigote stage followed by the meta cyclicbecause it causes skin sloughing. Trypan blue (infective) stage where division ceases. Thesestains the mucous membranes and other tissues metatrypanosomes are injected into the animalblue and is, therefore, not used commonly. host during a blood meal and become the blood- stream form of the organism. Trypanosomes doEfluvane not have a sexual stage of reproduction.Efluvane is a derivative of acridine, a non-specific Horses are susceptible to several Trypanosomabacteriostatic compound. It is an orange-red spp. with various transmission vectors:crystalline powder that is slightly soluble in saline. • tsetse fly: T. brucei, T. congolense, T. vivax;It is thought to intercalate with, and, therefore, • mechanical: T. evansi (surra), T. vivax;interfere with, the normal coiling of parasite DNA. • venereal: T. equiperdum (dourine).For premunition (this compound does not elimi-nate Babesia spp.), a 5% solution of efluvane isadministered i.v, at 4-8 ml/100 kg up to a maxi- PATHOGENESIS, CLINICAL SIGNSmum dose of 20ml. AND DIAGNOSIS In tsetse fly transmitted trypanosomiasis, metatry-Buparvaquone/parvaquone panosomes are injected into the skin of the animal by the tsetse fly during a blood meal and causeThe similarity of B. equi to Theileria spp. has led localized swelling. The trypanosomes migrate tomany investigators to look at the efficacy of some the lymph nodes and then into the bloodstream,theilericidal drugs in the treatment of B. equi where they undergo rapid asexual multiplication.infections. Preliminary studies suggest that these The clinical signs, incubation period and mortalitydrugs may be effective in eliminating B. equi rate vary with the Trypanosoma spp. involved.when used in combination with imidocarb. Equids are highly susceptible to T. brucei infec- tion, with a very high mortality rate within 14-90 days in untreated horses. The prepatent period, theTRYPANOSOMIASIS period from infection to parasitemia, is approxi- mately 6-10 days. Locomotor ataxia is the earliestProtista Sarcomastigophora Zoomastigophorea clinical sign. Remittent high fever (41.7°C/107°F)Kinetoplastida Trypanosomatidae Trypanosoma. occurs between intervening periods when the
  • 56. 54 EQUINE CLINICAL PHARMACOLOGYrectal temperature is normal. Anemia and icterus Chronic wasting occurs in the face of a normalare evident in the early stages of the disease. appetite and progressive anemia is evident.Depression, unthriftiness, lymphadenopathy, Urticaria is irregular and can be localized or gen-penile prolapse, urticarial plaques, edema and eralized. Lymphadenopathy, icterus and edemakeratitis are also seen. are also seen. The mortality rate is high and occurs Disease caused by T. congolense is rare in horses. in days to months. The course of disease is moreThe prepatent period is 17 days and the parasit- chronic in donkeys and mules. Ataxia, from corticalemia is unimpressive. Edematous plaques become involvement, occurs late in this disease.evident after about 10 days, and may recur during Dourine (T. equiperdum) is a venereal disease ofthe infection. Generalized edema becomes evident horses that is limited in geographical distributionfrom about 14 days postinfection. Keratitis may to Africa, the Middle East, southern and easterndevelop. T. congolense infection is relatively mild in Europe, Russia and Central and South America.horses: normal appetite despite a moderate anemia This trypanosome passes through intact mucousand clinical signs that resolve spontaneously in membranes. The incubation period is from 2 weeksabout 6 weeks. to 3 months or more. The infection is chronic and Equine disease caused by T. oioax is usually, but persists for years with a mortality rate of aboutnot always, relatively innocuous. The prepatent 50% within 2 years. The clinical signs of dourineperiod is about 16 days. Parasitemia is cyclic and develop over a period of weeks to months andis accompanied by fever. Between these febrile are more variable than in the other forms of try-episodes, urticarial plaques may become evident panosomiasis. The signs may be more evident inin the skin and dependent edema develops. In some locations than others: for example, the mostmore severe cases, ataxia may develop in the first evident sign may be in the genitalia or the centralfew days after infection, with muscle tremors nervous system (CNS) without significant clinicaloccurring around 2 weeks after infection. The signs elsewhere. Most commonly fever, swellingataxia and anorexia are progressive. Keratitis and edema, anemia, wasting, ocular lesions, ataxiawith associated corneal opacity may develop. and facial paralysis are seen, with the appetite Surra or murrina (T. eoansi also known as remaining normal. The earliest signs are edemaT. equinum) is transmitted mechanically. The most of the external genitalia and mucopurulent dis-significant effect of this mode of transmission is charge from the urethra of affected stallions orthat the disease is not limited to the geographical the vagina of affected mares. Fever is intermittentrange of the tsetse fly. This disease can be found and is more common in the early stages of thisin North Africa, the Middle East, Asia and disease. As the disease progresses, a skin rashCentral and South America. The genera of blood- consisting of large plaques (2-10 em in diameter)sucking flies that may act as mechanical vectors, develops. In the late stages of dourine, progres-by transferring blood on their mouthparts from sive paralysis occurs, starting with the face andan infected animal to a new host, include Tabanus, neck and progressing to the pelvic limbs.Stomoxys, Atylotus and Leperosia spp., although Abortion is also observed. The mortality rate isTabanus spp. is the most commonly implicated. high in untreated horses, while donkeys andThe vampire bat (Desmodus rotundus) is also mules may be infected without developing obvi-responsible for transmission in South America. ous clinical signs. Treatment is not recommendedThe pathogenesis, clinical signs and treatment unless the infected equine is not going to be usedof T. evansi are similar to those of the tsetse- for breeding since these horses may remain atransmitted trypanosomiases. The prepatent/ source of infection for other horses.latent period is highly variable but is probably The diagnosis of trypanosomiasis is based onapproximately 4-7 days. There is a reaction at the direct demonstration of the parasite or onthe site of the infective bite. Parasitemia is accom- serology. Direct detection includes the examina-panied by fever, which is sometimes extremely tion of blood smears, anion-exchange chromatog-high, ataxia and occasionally excessive thirst. raphy or mouse inoculation. The latter method is
  • 57. 3. ANTIPROTOZOAL DRUGS IIInot effective in the diagnosis of T. equiperdum resistance as a result of extensive drug use. It isbecause it rarely produces hemoparasitism. less effective against T. brucei but is used for theHowever, direct smears of fluid from the edema- treatment of T. congolense, T. vivax, T. evansi and T.tous genitalia of affected animals may reveal these equiperdum. For the treatment of T. brucei, T. evansitrypanosomes. Alternatively, a centrifuged sample and T. equiperdum infections, a 5% solution ofof blood may reveal parasites. Indirect serological quinapyramine sulfate is administered to horsestests including latex agglutination tests, enzyme- at 3-5mg/kg at 6h intervals, by s.c. or deeplinked irnmunosorbant assays (ELISAs) and com- i.m. injection, split between three injection sites.plement fixation tests are available. The last is of There may be edema and skin sloughing atparticular benefit in the diagnosis of T. equiperdum. the injection site after s.c, injection and these reac- tions are often marked and resolve very slowly (many months). Systemic signs of toxicity includeTREATMENT intense hypersalivation, dyspnea, hyperhidrosis,The treatment of trypanosomiasis in horses is colic and sometimes collapse. The systemictaken directly from the treatment regimens used reactions are most common is young animalsin other species. The choice of drug and the and may occur within a few minutes of drugadministration route depend upon the manage- administration.ment and chemosensitivity of the trypanosome Quinapyramine prosalt is a solution containingstrain in question. The recent development of 10% (three parts) sulfate and 6.67% (two parts)in vitro assays to determine trypanosome chemo- chloride that is administered s.c. at 0.025ml/kg.sensitivity mean that the choice of drug can be The chloride salt is not very soluble and producesbased on local drug resistance patterns. a s.c, deposit of the drug that is then absorbed slowly from the injection site, providing a pro- phylactic effect that lasts for up to 3 months. TheAromatic diminazenes addition of suramin markedly enhances the pro- phylactic effect of the quinapyramine. In endemicThe use of this class of drugs for the treatment of areas, a combination of quinapyramine prosalttrypanosomiasis is included in the section on and suramin is administered to stallions every 90piroplasmosis. days during the breeding season and to mares at least 18 days before breeding to protect againstQuinapyramine salts dourine (T. equiperdum) infection.The mode of action of this class of drugs is relatedto the inhibition of cell growth and division. Suramin Suramin is a synthetic complex aromatic organic compound. The sodium salt is a white or pinkishQuinapyramine crystalline powder with some solubility in water.Quinapyramine sulfate and chloride are bitter, It is useful in the treatment of T. brucei, T. evansiodorless, white to pale yellow crystalline pow- and T. equiperdum. Suramin markedly enhancesders. The sulfate salt is readily soluble in water the prophylactic effect of quinapyramine and theand the chloride salt is soluble in boiling water phenanthridinium compounds, although it has noup to a concentration of 2%. The sulfate salt has a prophylactic effects when used alone. It is mostrapid onset of action but a short duration effect, effective when given in the early stages of the dis-whereas the chloride salt has a long-acting effect ease because it attacks the Trypanosoma spp. as theyand is used for prophylaxis. concentrate in the lymph nodes and circulate in Quinapyramine sulfate is the most effective the bloodstream. This drug is less effective dur-treatment for trypanosomiasis in horses but is ing the later stages of the disease when the try-often poorly tolerated and there is widespread panosomes invade the eNS.
  • 58. 58 EQUINE CLINICAL PHARMACOLOGY A 10% solution of suramin sodium is adminis- 0.25-1 mg/kg bwt. It is effective against T. vivaxtered i.v, to horses at 7-lOmg/kg and the at O.5mg/kg i.m. and T. brucei and T. congolensetreatment repeated up to three times at weekly at 0.5-1 mg/kg i.m. It has prophylactic activityintervals. This drug is generally well tolerated for up to 6 months but is commonly administeredbut systemic reactions may occur after i.v, admin- as frequently as every 2 months. This drug causesistration. Edema, urticaria and laminitis are among severe local reactions and should be given bythe systemic reactions observed and prolonged deep i.m, injection divided between several injec-use may result in chronic nephritis with albumin- tion sites. Other transient toxic effects includeduria. Local reactions occur after i.m, administra- nasal discharge, flatulence, hypersalivation andtion. The widespread existence of stable suramin prostration. The side-effects observed in horsesresistance among trypanosomes has rendered this are substantial and warrant careful deliberationcompound of little use. when considering using this drug. Drug resist- ance occurs in regions where this compound is used widely for prophylaxis.Phenanthridinium compoundsHomidium bromide Aromatic diamidinesHomidium bromide is available as purple tablets Diminazene aceturate is used for the treatment(250 mg) that dissolve in boiling water. The drug of T. brucei infections in horses and minimalis prepared as a 1-2.5% solution that is admin- resistance is reported (see Piroplasmosis, p. 51).istered s.c. or i.m, at 1 mg/kg to horses. This dose However, it should only be used in horses whenis both prophylactic (for at least 1 month) and no other treatment is available because it pro-curative in susceptible trypanosome infections. duces severe local reactions (muscle necrosis). IfEthidium bromide can produce severe local reac- diminazene aceturate is used, the dose should betions, which are less pronounced following deep divided into two or three portions administeredi.m, injection. This compound is most effective 4h apart.against T. vivax, has lower efficacy againstT. brucei and T. congolense and no activity againstT. evansi. There is Widespread resistance to this Treatment recommendationscompound in tsetse regions. Isometamidium 0.5 mg/kg administered by deep i.m. injection divided between three injectionPyrithidium bromide sites is used for the treatment of T. congolense andPyrithidium bromide (prothidium) is available as T. vivax in horses. A 5% solution of quinapyraminered tablets (500mg) that are dissolved in boiling 3-5 mg/kg administered by s.c, or deep i.m,water to make a 2.5% solution (1 tablet per 10 ml injection divided between three injection sites andwater). It is administered to horses by deep i.m, administered as divided doses (three doses giveninjection at 2-2.5 mg/kg and confers protection at 6h intervals) is used for the treatment offor 4 months. Systemic reactions are rare but T. brucei and T. evansi in horses.severe local reactions may occur. This drugrapidly induces trypanosomal resistance, which Prophylaxis recommendationsincludes cross-resistance to quinapyramine andethidium. Isometamidium 0.5-1 mg/kg administered by deep i.m. injection into the neck muscles and withers, divided between several injection sites,Isometamidium chloride protects horses for 2--4 months against T. congolenseIsometamidium chloride hydrochlorate is a red and T. vivax. Prothidium bromide administeredpowder that is readily soluble in water. A 1-2% by deep i.m, injection 2 mg/kg protects horsessolution is administered by deep i.m. injection at for at least 3 months against T. congolense and
  • 59. 3. ANTIPROTOZOAL DRUGS 157T. vivax. Quinapyramine prosalt 7.4mg/kg pro- into trophozoites, which increase in size to formvides 3 to 4 months protection against T. congo- schizonts; these divide into many daughter cellslense, T. vivax and T. equiperdum but causes severe called merozoites. The merozoites are releasedlocal reactions. Suramin-quinapyramine complex from the infected intestinal cells and infect adjoin-administered s.c. at a dose rate of 10 mg/kg ing cells to continue the asexual reproductive cycleprovides protection for at least 6 months against (schizogony). After one or more cycles of schizo-T. evansi. gony, the merozoites differentiate into the sexual forms, the micro- and macrogametocytes, for sex- ual reproduction or gametogony. The final prod-GIARDIASIS uct of the sexual reproductive cycle is the oocyst or egg, which is then shed in feces and is infectivePARASITE BIOLOGY to new hosts. The prepatent period, the time from the ingestion of sporulated oocysts to the produc-Giardia spp. are intestinal parasites that are found tion of infective oocysts, is 16-35 days.in horses of all ages but rarely produce clinicalsigns. Infection is found commonly in foals of2-22 weeks of age (17-35%) and less commonly PATHOGENESIS, CLINICAL SIGNSin older horses. On rare occasions, chronic diar- AND DIAGNOSISrhea has been associated with giardial infection. A Coccidia are found in the small intestine of youngdiagnosis of giardiasis requires the identification horses and donkeys around the world. They areof Giardia spp. cysts in feces, usually using the found commonly in the feces of normal foalszinc sulfate centrifugal flotation method. The aged 30-125 days, suggesting that this organismshedding of cysts is inconsistent and, therefore, does not usually cause diarrhea or other clinicalfresh fecal samples should be tested daily for 5 signs in foals. However, severe diarrheic episodesdays. Other causes of diarrhea should be ruled have been attributed to massive coccidial infec-out before Giardia infection is diagnosed since tion in foals. A diagnosis of coccidiosis is made byGiardia spp. is shed commonly in equine feces in examining fecal sediment for oocysts or by flota-the absence of clinical signs. tion of oocysts using saturated sucrose solution. Beforea diagnosis of intestinal coccidiosis is made,TREATMENT other causes of diarrhea must be excluded because coccidia are rarely pathogenic in horses.The nitroimidazole metronidazole 5 mg/kg orallythree times a day for 10 days is effective in thetreatment of equine giardiasis. The benzimida- TREATMENTzole anthelmintic fenbendazole is used in the The treatment of choice for coccidiosis is the sul-treatment of giardiasis in dogs and cats. fonamide antimicrobial agents (see Ch. 2). The sulfonamides disrupt folic acid and nicotinamide metabolism and coenzymes I and II by competingCOCCIDIOSIS (GLOBIDIOSIS) with para-aminobenzoic acid (PABA). Coccidia must manufacture their own folic acid and, there-PARASITE BIOLOGY fore, this step is mandatory in the pyrimidineEimeria leukarti is the intestinal coccidian of pathway of these parasites. Sulfamethazine (sul-horses. Infection with E.leukarti follows the inges- fadimidine) at 220mg/kg i.v, or orally, or sul-tion of sporulated oocysts in contaminated food fadimethoxine (55mg/kg) orally or sulfathiazoleor water. After exposure to bile in the small intes- (66mg/kg) orally, all once daily for 5-7 days, aretine, the oocysts excyst and individual protozoa, used commonly for the treatment of equine coc-the sporozoites, emerge. The sporozoites pene- cidiosis. The signs of toxicity of the sulfonamidestrate the intestinal epithelium and differentiate are covered extensively in Chapter 2. Crystalluria,
  • 60. 58 EQUINE CLINICAL PHARMACOLOGYa sign of toxicity in other species, is uncommonly feed, hay or pasture that is contaminated withobserved in horses. feces of the definitive host; the route of infection of the natural intermediate host. The sporocysts are ingested, excyst, penetrate enterocytes and ulti-SARCOCYSTIS INFECTIONS: mately enter the circulation, but they apparentlyEQUINE PROTOZOAL never encyst in the tissues of the horse. Instead,MYELOENCEPHALITIS in some horses, they migrate to the CNS and con- tinue to undergo schizogony intracellularly inPARASITE BIOLOGY neurons and microglial cells without forming tis-Equine protozoal myeloencephalitis (EPM) is sue cysts. The merozoites are found free in thecaused by Sarcocystis neurona (the most recognized cytoplasm of cells in the CNS, suggesting that thecause) or by Neospora hughesi (possibly transpla- merozoites of S. neurona never mature beyondcentally). Sarcocystis spp. has a heteroxenous (two what would normally be the second generationhost species) life cycle with gametogony (sexual of division. Horses cannot transmit S. neurona toreproduction) taking place in the definitive host other animals, including other horses.(usually a predator or scavenger species) and The time from exposure to S. neurona to theschizogony or merogony (asexual reproduction) development of marked clinical signs of EPM isin the intermediate host (usually a prey species). highly variable. The disease has been identifiedSexual division begins in the intestinal tract of in a 2-month-old foal and there is no evidencethe definitive host within 18h of the ingestion of that transplacental transmission occurs, suggest-cysts in the muscle of the intermediate host. ing that 2 months may be around the minimumHowever, the time required for the development time required for the development of this diseaseof the sexual stages (gametogony) is not known. in horses. An older horse with acute onset spinalIngested bradyzoites rapidly penetrate entero- ataxia was seronegative (antibody) for S. neuronacytes and develop into the sexual stages of the in both serum and cerebrospinal fluid (CSF)whenorganism, the micro- and macrogametes. Motile the clinical signs of EPM developed, but becamemale microgametes penetrate the macrogamete seropositive in both fluids within 3.5 weeks of theto cause fertilization. Sporulation occurs within onset of these signs. This suggests that the mero-the oocyst in the enterocytes of the definitive zoites of S. neurona migrated into the CNS of thishost. Two sporocysts, each of which contains four horse before antibodies could be detected in theinfective sporozoites, form within the oocyst. bloodstream, which takes around 10-14 daysOocysts are passed in the feces of the definitive after the exposure to merozoites. Therefore, ithost and ingested by the intermediate host. Upon appears that horses may develop clinical signsexposure to bile in the duodenum of the interme- of EPM within only a few weeks of infection.diate host, or in the case of S. neurona the aberrant However, the development of clinical signs ofintermediate host the horse, the sporozoites EPM may require as long as 2 years, since it hasexcyst from the protective sporocysts. The sporo- been seen in horses that have been exported. Inzoites then enter the intestinal epithelial cells and fact, EPM has only been reported outside North undergo the first of many stages of asexual repro- America in exported horses, although there have duction (schizogony or merogony) to produce been no comprehensive studies to determinetachyzoites (merozoites). The merozoites undergo whether exposure to S. neurona occurs in otherseveral cycles of replication in this manner. countries. The clinical signs of EPM are the result of both direct damage to neurons, by protozoal prolifera-PATHOGENESIS, CLINICAL SIGNS tion within the neuronal cell bodies, and indirectAND DIAGNOSIS damage to neural elements, produced by edemaHorses are presumed to become infected with S. and inflammation in response to the merozoitesneurona by the ingestion of infective sporocysts in and meronts in the CNS. Deposition of the parasite
  • 61. 3. ANTIPROTOZOAL DRUGS 69within the CNS is presumed to occur by hemato- other in the brainstem. Facial nerve paralysis isgenous spread, because of the parasites predilec- associated with muzzle deviation away from thetion for endothelial cells. Although the spinal cord affected side, ptosis and ear droop. Vestibularseems to be the most commonly affected region, signs, including nystagmus, and head tilt and athe protozoa may begin to proliferate and cause wide-based stance can occur.dysfunction at any site in the CNS. The disease Infection of the cerebrum, basal ganglia andis often insidious in onset, misdiagnosed until cerebellum are observed less commonly. Depres-late in its course and may culminate in death if sion is associated with other cerebral abnormalitiesuntreated. but is uncommon in EPM. Protozoal infections The classic presentation for EPM is progres- in the cerebrum may be focal and associatedsive asymmetric ataxia and focal muscle atrophy. with seizure activity and electroencephalographicAlthough the most common presenting sign of abnormalities. Alternatively, asymmetric amau-EPM is ataxia (incoordination), any neurological rosis (central blindness) and facial hypalgesia maydysfunction or even lameness can occur. In fact, be observed. Infection of the cerebellum results inthe neurological signs that occur are referable to cerebellar ataxia that is usually not associated withthe site or sites of infection. Both white and gray weakness or proprioceptive deficits. Involvementmatter damage, resulting in upper and lower of the reticular activating system is also uncom-motor neuron signs, respectively, produce locomo- mon but may produce a narcolepsy-like syndrometor deficits. Upper motor neuron damage results in the absence of any other neurological signs.in ataxia and spasticity while lower motor neu- Occasionally, lameness that cannot be eliminatedron damage results in dragging of one or more with nerve and joint local anesthesia may be thetoes and weakness. There is no consistent pattern only evidence of a neurological deficit.of gait deficits found in horses with EPM and Low numbers of protozoa in the CNS producequadriplegia may be present in severe cases. this disease, rendering direct methods of parasite Signs of cranial nerve dysfunction are seen in detection of little use in the diagnosis of EPM.at least 10% of cases of EPM. Any cranial nerve Therefore, it is diagnosed indirectly using serologynucleus (collection of nerve cell bodies demarcated (immunoblot). Both serum and CSF is tested, withwithin the CNS) may be affected if the infection is a positive serum antibody titer indicating exposurein the brainstem. Airway abnormalities, such as to the parasite and a positive CSF titer indicatinglaryngeal hemiplegia (vagus nerve) or dorsal dis- either blood contamination of the CSF duringplacement of the soft palate (glossopharyngeal sampling or intrathecal antibody production.nerve), may result from infection of the nuclei ofthe respective cranial nerves. Most horses with air- TREATMENTway abnormalities do not have EPM; therefore, adiagnosis of EPM may be overlooked. Atrophy of Potentiated sulfonamidesthe temporalis or masseter muscles (trigeminal (pyrimethamine plus sulfadiazine)nerve) may be observed. This can be accompanied The recommendations for the treatment of EPMby dysphagia, which may also be caused by abnor- using pyrimethamine, trimethoprim and sulfadi-malities of the glossopharyngeal, hypoglossal or azine were originally based on the use of thesevagus nerves. Difficulty in the prehension, masti- drugs for the treatment of malaria and toxoplas-cation and deglutition of food may be difficult to mosis in humans. Either pyrimethamine or tri-assess unless the horse is observed eating. Evi- methoprim in combination with sulfadiazine ordence of quidding (dropping chewed feed) or sulfamethoxazole have been used with some suc-aspiration or reflux of chewed material into the cess and have gained widespread acceptance asnostrils may occur as a result of cranial nerve dys- the treatment of choice for EPM. Pyrimethaminefunction. Abnormalities of the facial and vestibu- and trimethoprim are diaminopyrimidine antimi-locochlear nerves are often observed together crobial agents that inhibit dihydrofolate reduc-because of the proximity of these nuclei to each tase (DHFR; see Ch. 2). These agents interfere with
  • 62. 80 EQUINE CLINICAL PHARMACOLOGYthe production of the enzyme cofactor tetrahydro- additive effects on mammalian DHFR (Burchallfolate from dihydrofolate. Diaminopyrimidines 1973). The most commonly observed sign of toxi-interfere with both the de novo production of city of pyrimethamine and sulfadiazine combina-tetrahydrofolate and the recycling of this cofactor, tions is bone marrow suppression (see Ch. 2).interfering with bacterial and protozoal DNAsyn- This effect causes a gradual increase in red bloodthesis. The antimicrobial effect of the diaminopy- cell size during treatment, anemia and neutrope-rimidines is potentiated by the addition of a nia, which can be profound (Fenger 1997). Rarely,sulfonamide agent. the chronic administration of pyrimethamine and The sulfonamides are analogs of PABA sulfadiazine, particularly at higher than the rec-that compete in the production of dihydrofolate ommended dose rates, also causes a neurologicalby the enzyme dihydropteroate synthase (see Ch. syndrome. This syndrome includes symmetric2). Sulfadiazine is the sulfonamide of choice for ataxia, dysphagia and bilateral facial nerve paral-the treatment of EPM because it penetrates the ysis in addition to anemia and neutropenia (Polk,CNS better than other sulfonamides, producing unpublished data, 2003). Incoordination has alsoconcentrations of 10-60% of serum concen- been reported in a study where healthy poniestrations (Boger 1959, Shoaf et al1989). In general, were given pyrimethamine with trimethoprimmost protozoa are resistant to the sulfonamides and sulfamethoxazole (Bedford & McDonnellbut the effects of these drugs are greatly enhanced 1999); however, this effect has not been observedby the presence of pyrimethamine. Horses in clinical cases. This side-effect may confuse theshould remain on both drugs for the duration of assessment of the neurological signs of EPM.treatment. Apicomplexan protozoa that are ordi- However, this neurological syndrome is alwaysnarily susceptible to pyrimeth-amine, including accompanied by bone marrow suppression and,Plasmodium spp. (malaria), have been shown to therefore, a complete blood count may help tobecome rapidly resistant to pyrimethamine in the confirm or exclude pyrimethamine toxicity as theabsence of the sulfonamides (Watkins & Mosobo cause of the ataxia.1993). Based upon pharmacokinetic information andminimal inhibitory concentrations (MIC) for Benzeneacetonitriles (diclazuril)pyrimethamine, an oral daily dose of 1 mg/kg The benzeneacetonitrile diclazuril has been usedin combination with 22 mg/kg sulfadiazine is in the treatment of EPM in horses. Its mode ofrecommended in horses. The duration of treat- action is not completely understood but it isment is controversial and the recommendations believed to have efficacy against a plastid-likerange from a minimum of 3 months (least conser- organelle in protozoa. Oral administration ofvative) to the point at which the CSF is seronega- diclazuril to horses at 5.5 mg/kg for 21 daystive (most conservative). It is clear that the latter provides CSF concentrations of 100-250ng/ml,course of treatment is least likely to result in which exceed the concentrations required torelapse of the clinical signs (Fenger 1997),but it is inhibit 95% of the proliferation of S. neurona in tis-likely that this exceeds what is absolutely neces- sue culture (Dirikolu et al1999). This compoundsary in most cases. is currently undergoing safety and efficacy trials Trimethoprim should not be used in combina- in the USA for the treatment of EPM in horses.tion with pyrimethamine since both drugs areDHFR inhibitors, although pyrimethamine ismore selective for the protozoal enzyme and Symmetrical triazinonestrimethoprim for the bacterial enzyme. Whenused together, trimethoprim competitively inhibits Toltrazuril and ponazuril (toltrazuril sulfone) arepyrimethamine, thus decreasing the efficacy of water-soluble, coccidicidal agents that are effec-this more effective compound. In addition, tive against the schizogony stage of most coc-pyrimethamine and trimethoprim have at least cidia. Oral toltrazuril at 5 mg/kg once daily for
  • 63. 3. ANTIPROTOZOAL DRUGS 8160-90 days or, more frequently, lOmg/kg for foals. Nitazoxanide is currently undergoing safety28 days is recommended and appears to produce and efficacy trials in the USA for the treatment offew side-effects in horses. There are some veteri- EPM in horses.narians that use large doses administered bynasogastric tube 10 days apart. Unfortunately, OTHER SARCOCYSTIS INFECTIONSthere is a high rate of relapses associated withthis practice and the properties of this drug, such PARASITE BIOLOGYas drug half-life in the body, do not support thispractice. The dosage of ponazuril is 5 mg/kg Sarcocystis bertrami, S. equicanis and S. fayeriorally once daily for 28 days. Large doses of pon- use horses as their natural intermediate host.azuril produce few side-effects apart from uter- S. bertrami was first described in horses in 1901.ine edema, suggesting that the uterus may be one Subsequently, muscle cysts have been identifiedof the target organs for toxicity. Ponazuril has in horses in Germany, Austria, Morocco, the UKrecently been approved in the USA for the treat- and the USA; in donkeys in Sardinia, Egypt andment of EPM in horses. Morocco; and in a zebra in South Africa, perhaps representing two or three separate Sarcocystis spp. S. bertrami and S. equicanis can be distin-Nitrothiazoles (nitazoxanide) guished based on their unique cyst wall mor-Nitazoxanide is a nitrothiazolyl-salicylamide com- phology. Cysts of both species are macroscopic,pound that has a wide range of activity against ranging in length from 9-15 mm for S. bertrami tobacteria, intestinal parasites and protozoa. Its 1-9 mm for S. equicanis. S. fayeri is clearly a distinctmechanism of action is not known but the com- species: sarcocysts from this species are micro-pound is structurally related to the antimicrobial scopic and exhibit unique cyst wall morphology,agent metronidazole. This compound is unusual in smaller sporocysts and a longer prepatent periodthat it is effective against gastrointestinal proto- than S. bertrami. Dogs serve as the definitive hostszoan and helminth parasites, Trichomonas vaginal is, for S. equicanis and S. fayeri.and some Gram-positive and Gram-negative bac- S. fayeri infection has been estimated to occurteria. It has also been reported to be effective in in as many as 88% of horses in Chile and as fewtreating metronidazole-resistant giardiasis. It is as 30% of horses in the USA. This parasite encystshighly lipophilic and, therefore, has greater in microscopic cysts that may be up to 990 urnbioavailability when administered with oil or oil- long and 136 urn wide. Sporocysts are ingestedcontaining feeds, such as typical equine grain and two generations of schizogony take place inrations. Nitazoxanide is marketed throughout the endothelial cells of the arteries or capillariesmost of Latin America for use in treating intes- in the heart, brain and kidney 10-25 days post-tinal parasitism in humans. It is currently under infection. After several cycles of replication in theregistration in other parts of the world and is horse, the merozoites enter the myocytes of theavailable ("orphan drug" status) in the USA for esophagus, tongue and skeletal muscles and formthe treatment of cryptosporidiosis inpatients with sarcocysts. By the 77th day postinfection, the sar-acquired immunodeficiency syndrome (AIDS). cocysts are infectious to the definitive host, thePreliminary studies indicate that 77% of horses dog. Once a dog ingests horse muscle containingtreated with nitazoxanide paste, either at 25 mg/kg sarcocysts, infective sporocysts are shed after afor 5 days followed by 50 mg/kg for an addi- prepatent period of 12-15 days. Transplacentaltional 23 days or at 50mg/kg for 28 days, infection of S. fayeri also occurs.improved with treatment. Side-effects occur in All three Sarcocystis spp. identified so far areless than 15% of horses and include fever, diar- minimally pathogenic in horses. Natural infectionsrhea, colic, limb edema and laminitis. Pregnant are often found incidentally during postmortemmares have been treated without any pregnancy- examination. Feeding studies suggest that thererelated problems or the production of abnormal are both dose- and strain-specific factors that
  • 64. 62 EQUINE CLINICAL PHARMACOLOGYdetermine sporocyst pathogenicity. For example, a REFERENCESpony fed 1 million sporocysts of S.fayeri developedmild transient anemia and fever, while a horse Bedford S J, McDonnell S M 1999 Measurements of reproductive function in stallions treated withfed 10 million sporocysts of S. fayeri developed a trimethoprim-sulfamethoxazole and pyrimethamine.stiff gait. Another pony fed 2 million sporocysts Journal of the American Veterinary Medical Association 215:1317-1319from a different source developed lethargy, depres- Boger W P 1959 The diffusion of sulfonamides into thesion and unthriftiness 150 days after infection. A cerebrospinal fluid: a comparative study. AntibioticGerman isolate of S. fayeri was even more patho- Medicine and Clinical Therapy 6:32-40 Burchall J J 1973 Mechanism of action of trimethoprim-genic; causing similar clinical signs in five ponies sulfamethoxazole. Journal of Infectious Diseasesfed only 200000 sporocysts each. Myositis has 128(suppl):437-441been associated with the presence of sarcocysts in Dirikolu L, Lehner F, Nattrass C et al 1999 Diclazuril in the horse: its identification and detection and preliminaryequine muscle; however, inflammation is rarely pharmacokinetics. Journal of Veterinary Pharmacologyfound in association with the muscle cysts. None- and Therapeutics 22:374-379theless, in some cases, inflammation and increased Fenger C K 1997 Equine protozoal myeloencephalitis. Compendium on Continuing Education for the Practicingserum concentrations of creatine phosphokinase Veterinarian 19:513and aspartate aminotranferase are found in asso- Melhorn H, Schein E 1998 Redescription of Babesia equiciation with the tissue cysts. Laveran, 1901 as Theileria equi. Parasitology Research 84:467-475 Shoaf S E, Schwark W S, Guard C L 1989 Pharmacokinetics of sUlfadiazine/trimethoprim in neonatal male calves:PATHOGENESIS, CLINICAL SIGNS effect of age and penetration into cerebrospinal fluid.AND DIAGNOSIS American Journal of Veterinary Research 50:396-402 Taylor W M, Simpson C F, Martin F G 1972 Certain aspectsNatural disease caused by Sarcocystis spp. is of toxicity of an amicarbalide formulation to ponies.uncommon in horses, so diagnosis is rarely nec- American Journal of Veterinary Research 33:533-541 Watkins W M, Mosobo M 1993 Treatment of Plasmodiumessary. However, histopathological evidence of fa/ciparium malaria with pyrimethamine and sulphadoxine:inflammation associated with cysts in muscle a selective pressure for resistance is a function of longbiopsy samples would be highly suggestive of elimination half-life. Transactions of the Royal Society of Tropical Medicine and Hygiene 87:75-79sarcocystis-related myositis.TREATMENTOxytetracycline or pyrimethamine plus sulfadi-azine may be effective in eliminating Sarcocystisspp. cysts.
  • 65. CHAPTER CONTENTSIntroduction 63Endoparasiticides 64 Drugs 64 Parasiticides Control programs for internal parasites 67 Sandy Love Robert M. ChristleyEctoparasiticides 71 Drugs 71 Control programs for ectoparasites 72Miscellaneous parasiticides 72References 73 INTRODUCTION Since 1917, only 11 new endoparasiticides have been developed for use in the horse. Many of the early compounds had very narrow spectra of activity and/or high potential for toxicity such that they have become obsolete. Febantel, levamisole, trichlorfon, dichlorvos, phenothiazine and carbon disulfide are no longer used routinely in the horse (Lyons et aI1999). Following the landmark studies of Drudge & Lyons (1966),the concept of interval anthelmintic dosing has been the mainstay of equine parasite control programs. Since the initial efficacy reports for drugs against equine parasites-benzimida- zoles (Drudge et al 1963), pyrantel (Cornwell & Jones 1968), ivermectin (DiPietro et al 1982) and moxidectin (Lyons et aI1992)-intensive, interval dosing with these potent, broad-spectrum anthelmintics has been practiced widely, which has resulted in major changes in clinical para- sitism. Cyathostomes have superseded Strongylus vulgaris as the major equine parasitic pathogens (Love et al 1999) and the results of quantitative epidemiological studies have provided evidence that tapeworms, previously considered a minor pathogen, are important in the etiopathogenesis of certain forms of colic (Proudman et al 1998). In essence, this reflects the excellent efficacy of mod- em anthelmintics against S. vulgaris, which now rarely infects well-managed horses (Drudge & Lyons 1986) and highlights the generally poor effi- cacy of the same compounds against early mucosal stages of cyathostomes. Concurrent with widespread application of intensive interval anthelmintic dosing, there has 63
  • 66. 84 EQUINE CLINICAL PHARMACOLOGYbeen worldwide documentation of benzimidazole usage in equids, namely the macrocyclic lactones,resistance in cyathostome populations and, on a benzimidazoles and pyrimidines (Table 4.1). Inmore limited scale, resistance has also been addition, two compounds with a narrow spec-reported to piperazine and pyrantel (Lyons et al trum of activity, piperazine and praziquantel, are1999). To date, the only class of equine used in combination preparations, with theanthelmintics that cyathostomes have not devel- benzimidazoles and ivermectin, respectively. Eachoped resistance to are the macrocylic lactones class of anthelmintic has a discrete mode of action,(ivermectin and moxidectin) but Sangster (1999) which dictates the spectra of parasites on whichforecasted that this would occur by 2004. the anthelmintics will have toxic effects (MartinImportantly, there is no evidence of reversion to 1997). Anthelmintic efficacy depends on both thesusceptibility after protracted period of with- presence of specific drug receptors within thedrawal of benzimidazoles from parasite control parasite and on achieving sustained high concen-programs (Uhlinger & Johnstone 1984). trations of anthelmintic at the location of the par- With regard to ectoparasiticides, public health asite within the host tissues (Lanusse & Prichardand environmental concerns have led to the with- 1993). The absorption, distribution and elimina-drawal of the organochlorines and organophos- tion of anthelmintic compounds can be affectedphates in many countries. Since Elliot (1973) by dosage formulation, route of administrationreported the first photostable synthetic pyrethroid, and animal species (Baggot & McKellar 1994).these compounds have both replaced the naturallyoccurring pyrethrins (extracted from chrysanthe-mum flowers) and progressively become the DRUGSmainstay of external parasite control programs. Macrocyclic lactones Overall, much more is known about the phar-macology of endoparasiticides than is known There are five macrocyclic lactones utilized inabout the biology of the target parasites. With veterinary practice of which ivermectin (an aver-development and marketing of new anthelmintics mectin) and moxidectin (a milbemycin) arewith novel modes of action extremely unlikely in approved for use in the horse. Ivermectin andthe foreseeable future (Hennessy 1997), and moxidectin selectively paralyze parasites byagainst the background of the eventual possibility increasing muscle chloride permeability throughof resistance to ivermectin and/or moxidectin interaction with glutamate-gated chloride ion(Sangster 1999), education in the clinical pharma- channels (McKellar & Benchaoui 1996). There iscology and responsible use of equine anthel- evidence that ivermectin and moxidectin maymintics in parasite control programs are major have several sites of action within the parasiteissues in veterinary medicine. With regard to and the effects of these compounds may also dif-equine ectoparasites, there is huge regional and fer between the nematode species (McKellar &international variation in their importance both as Benchaoui 1996, Martin 1997).primary causes of disease and also as vectors of Macrocyclic lactones are highly lipophilic andother microbial pathogens. Equine protozoal are distributed widely to and eliminated slowlymyeloencephalitis (EPM), a major disease of the from the body compartments such that they havehorse, has created many issues for clinicians with persistent anthelmintic activity. There are physic-regard to achieving successful therapeutic proto- ochemical differences between ivermectin andcols; these are addressed in Chapters 3 and 9. moxidectin that confers different lipid solubility on each drug: moxidectin has a much longer half- life in body fat (Afzal et al 1997). This constitutesENDOPARASITICIDES at least a partial explanation for the longer period of suppression of equine fecal worm egg outputThree principal chemical classes of anthelmintic achieved following dosing with moxidectinwith a broad spectrum of activity are in common compared with ivermectin (Jacob et al 1995,
  • 67. Table 4.1 Pharmac01ogicaI and therapeutic features of equine antheIminticsClass Anthelmintic Administration Dose Mode of Duration of Lethal to major parasites route (mglkg)" action action (weeks)b LS MLS SS SSML T A BMacrocyclic Iactones Ivermectin p.o. 0.2 Affect glutamate- 6-8 .I .I .I .I .I gated chloride ion channels Moxidectin p.o. 0.4 13 .I .I .I .Ic .I .IdBenzimidazoles Fenbendazole p.o. 5-10" Beta-tubulin binding 4--6 .I .fS .Ie.! .I .Ie _ and inhibition of microtubule formation Mebendazole p.o. 8.8 4--6 .I .t1 OXibendazole p.o. 10-15 4--6 .I .t1 Oxfendazole p.o. 10 4--6 .I .19 .t1 Tiabendazole p.o. 44 4--6 .I .r .t1Pyrimidines Pyrantel p.o. 6.6 (13.2)i Cholinergic effect on 4 .I .t1 pamoate parasite ganglia Pyrantel p.o. 19 (32)i 4 .I .t1 embonate Pyrantel tartrate p.o. 2.6 i Continuous .I .IC .t1Heterocyclics Piperazine p.o. 88 Neuromuscular 4--6 .Ii .I HyperpolarizationPyrazinoisoquinolines Praziquantef p.o. 1.5 Increased ca2 + Unknown permeabilityp.o., oral; S.C., subcutaneous; LS. adult large strongytes; MLS. migrating large strongyle larvae; SS. adult small strongyles (cyathostomes); SSML, inhibited mucosal larval small strongyles;T, tapeworm; A, ascarids; S, bois"Regional differences exist in recommended dose rates and label claimsbQuration of action is the time from dosing until worm eggs appear in feces, i.e. the so-called egg reappearance period (ERP)CJO-.4O% efficacy~efficacy"Standard dose rate in USA is 5mg/kg; standard dose rate in Europe is 7.5mg/kg. ascarid dose is 10mglkg. MLS dose is either 10mglkg (7.5mglkg. in Europe) on 5 consecutive days or50 mgIkg on 3 consecutive days or single dose 60 mglkg. SSML dose is 10 mg/kg on 5 consecutive daysfAntheimintic resistance may affect efficacyll6O-75% efficacyhMLS dose is 440 mgIkg on 2 consecutive days; ascarid dose is 88 mg/kg$tandard dose rate in USA is 6 mglkg pyrantel pamoate: standarddose in Europe is 19 mgIkg pyrantel embonate; pyrantel tartrate dose is 2.6 mglkg daily in feed; tapeworm dose is twice standard dose50% efficacy"The mode of action results in rapid death of parasites such that they may cause intestinal obslructionIrupture if given to animals with high burdens of Parascaris equorumCombination product with ivermeclin
  • 68. 68 EQUINE CLINICAL PHARMACOLOGYTaylor & Kenny 1995, DiPietro et al 1997, the micro filariae of Onchocerca cervicalis (FrenchDemeulenaere et al 1997). However, while there et al 1988). In ruminants and dogs, macrocylicare significant differences in the pharmacokinetic lactones are marketed as endectocides, reflectingprofile (longer plasma residence time and higher their activity against both nematodes and arthro-peak plasma concentrations) following oral dosing pods. However, the formulations indicated forwith commercial preparations of moxidectin use in the horse have no label claim for efficacyand ivermectin, this may also reflect differences against equine lice, mites or ticks.in the product formulation (oral gel versus oral Generally, the exceptional antiparasitic potencypaste) and manufacturers recommended dose of macrocyclic lactones (in the microgram per kilo-rates (O.4mg/kg versus 0.2mg/kg) as well as the gram body weight range) renders them extremelydifferent lipophilicity of the two compounds safe in mammals; toxic doses are normally in the(Perez et al 1999). tens of milligrams per kilogram ranges (McKellar There are diverse formulations and delivery & Benchaoui 1996). However, the particularlysystems for macrocylic lactones in ruminants, high lipophilicity of moxidectin can predisposeincluding injectable, oral, sustained-release bolus to toxicosis in young foals and in emaciated ani-and transdermal ("pour on") products. In Europe, mals with insufficient adipose tissue. This resultsit is popular clinical practice to administer in adverse neurological reactions, including pro-injectable solutions of ivermectin intravenously longed coma and, in some cases, death (Johnson(i.v.) to horses. This constitutes extra-label (unli- et al1999). Although the product label contains acensed) use and there are no objective data to sup- specific contraindication to the use of moxidectinport the perceived improved efficacy following in foals less than 4 months of age, this may beadministration by this route. Specifically, in rela- because of a lack of specific data in foals of this agetion to hypobiotic cyathostome larvae, Klei et al group rather than an observed toxicity.(1993) reported no increase in efficacy when horses The possibility of environmental effects ofwere administered 10 j.Lg/kg, which is five times ivermectin has been a contentious issue sincethe recommended dose rate. Wall & Strong (1987) reported differences in rate Ivermectin and moxidectin have a broad spec- of degradation of fecal pats from treated com-trum of activity and high efficacy against the adult pared with untreated ruminants. There are nostages of the major parasites of the horse with the published data on similar studies in Equidae fol-notable exception of the tapeworm (Costa et al lowing oral administration of either iverrnectin1998). However, ivermectin has only limited or moxidectin.activity against fourth-stage cyathostome larvae.Although moxidectin has in the region of 60% Benzimidazolesactivity against late third-stage and fourth-stagecyathostomes, neither ivermectin nor moxidectin Currently, four benzimidazole compounds are inare consistently effective against hypobiotic third- common use in equine practice: fenbendazole,stage cyathostome larvae, with reported effica- oxibendazole, mebendazole and oxfendazole.cies ranging from 10 to 90% (Eysker et al 1992, The benzimidazoles selectively bind to the nema-Klei et al 1993, Xiao et al 1994, Monahan et al tode l3-tubulin and inhibit the formation of micro- 1995, Bairden et a12001). Ivermectin has variable tubules, which are intracellular organelles with aactivity against fourth-stage ascarids (Campbell variety of functions including the movement ofet al 1989). There are marked differences in the both energy metabolites and chromosomes dur-activity against bots (Gasterophilus spp.larvae), with ing cell division and the provision of the skeletalivermectin 95%effective (Britt & Preston 1985)and structure of the cell (Martin 1997). The benzimi-moxidectin only 20% effective (Xiao et al 1994). dazoles essentially starve the nematodes viaIvermectin is also documented as being active intestinal cell disruption and inhibit worm eggagainst Strongyloides westeri (Ryan & Best 1985), production. From a clinical standpoint, the most Dictyocaulus arnfeidi (Britt & Preston 1985) and important aspect of the mode of action of the
  • 69. 4. PARASITICIDES 87benzimidazoles is that, compared with anthel- Piperazinemintic compounds that disrupt parasite neuro- Piperazine is a gamma-aminobutyric acid (GABA)transmission, the onset of the anthelmintic effect agonist that hyperpolarizes the muscle membraneis slow. The individual benzimidazoles have dif- potential and so increases membrane conductance,ferent absorption, distribution and elimination which produces spastic paralysis of the parasite.characteristics such that there are differences in Piperazine on its own has a narrow spectrumthe dose rates required to achieve comparable of activity against adult stages of cyathostomesantiparasitic efficacy. Furthermore, efficacy is and ascarids but is also marketed in a varietyachieved with the more-soluble benzimidazoles, of combinations with individual benzimidawhich are eliminated rapidly, by exposing para- zoles. Such combination products have a broadsites to lethal plasma levels for longer by admin- spectrum of activity and may be effective againstistering the drugs over a prolonged period benzimidazole-resistant cyathostomes, but sepa-(Prichard et al 1978, McKellar & Scott 1990). In rate piperazine-resistant cyathostome populationsthe horse, this has led to the development of spe- have also been identified (Britt & Clarkson 1988,cific "5 day" dosing protocols with fenbendazole Drudge et a11988, Pereira et aI1991).(Duncan et a11980, 1998, Lyons et aI1986). The benzimidazoles have a broad spectrum of Praziquantelactivity against adult strongyles (assuming sus-ceptible cyathostome populations) ascarids and Praziquantel is indicated for use in the horse inlungworm but not tapeworm and bots. Multiple several countries as a component of a combina-dosing strategies of fenbendazole are effective tion ivermectin product (Mercier et al 2001). Theagainst migrating S. vulgaris larvae, mucosal hypo- mode of action of praziquantel has been studiedbiotic third-stage cyathostome larvae, migrating extensively in trematodes (Harnett 1998) and theascarid larvae and lungworm (Clayton & Neave anticestodal effects of this compound are assumed1979, Duncan et al 1980, 1998, Lyons et al 1986, to have a similar basis. Praziquantel increases theVandermyde et al 1987). permeability of tegumental and muscle cells to calcium ions, which results in parasitic muscular contraction and paralysis. In the horse, prazi-Pyrimidines quantel has a narrow spectrum of activity againstThree pyrantel salts (pamoate, embonate and tar- tapeworm, with 89-100% efficacy at dose rates oftrate) are indicated for use in the horse. The pyrim- 0.75-1 mg/kg (Lyons et aI1992).idines are selective agonists at synaptic andextrasynaptic nicotinic acetylcholine receptors onnematode muscle cells, which produce spastic CONTROL PROGRAMS FOR INTERNAL PARASITESparalysis of the parasites. Pyrantel salts are active against adult and The fundamental concept of the control of equineluminal stages of strongyles and ascarids but internal parasites is to reduce the transmission ofhave only limited efficacy against migrating lar- parasites between animals. Although it is possibleval parasites and no activity against bots in the to achieve this by exclusively non-chemical means,stomach. At double dose rates, either 13.2mg such as either frequent collection of pasture fecalpamoate salt/kg (Lyons et al 1986) or 38mg pats (Herd 1986) or feeding grazing animalsembonate salt/kg, pyrantel salts are effective in nematode-trapping fungi (Larsen 1996), typicalthe treatment of tapeworm infections. Pyrantel control programs for internal parasites are basedtartrate, administered at a continuous low level on anthelmintic dosing (Table 4.2). The programof 2.6mg/kg in feed, is effective against adult should be designed on the basis of epidemiologi-strongyles and ascarids, as well as being active cal features of interest. Most commonly, these areagainst newly ingested infective third-stage larvae strongyles (especially cyathostomes), tapeworms(Valdez et aI1995). and ascarids (when young stock are present).
  • 70. 88 EQUINE CLINICAL PHARMACOLOGY Table 4.2 QuldeUnes for Internal parasite control programs Program Dosingregimen Comments Targeted dosing Monthly FWEC all grazing animals Appropriate on farms with: strictlycontrolled grazing management; mature horsesonly; minimal new intake animals; efficient and compliant owner/ manager prepared to uriaertake frequent sampling Doseall FWEC positive (or arbitrary Standard faecal tests haveverypoor sensitivity for level, say> 200epg) with principal tapeworm detection: modified test has been anthelmintic at "standard" dose rate developed that is 61% sensitive and 98% specific (Proudman & Edwards 1992) Biannual tapeworm faecal analysis Tapeworm quantitative serological test available in or tapeworm serology Europe Dosealltapeworm-positive animals with pyrantel pamoate 13.2mg/kg p.o. (USA) or pyrantel embonate 38 mg/kg p.o, (Europe) Strategic dosing Onlyspring/summer dosing Regional variations in climate affectthe total period All grazing animals dosed at same for which suppression of faecal worm egg output time points with principal anthelmintic is required Dosing with principal anthelmintics Appropriate on premises in areas which have repeated on one or two occasions at discrete periods of weather detrimental to parasite predetermined intervals (based on ERP survival on pasture associated with each drug class; seeTable 4.1) Interval dosing Year round synchronized dosing all Intensive usage anthelmintlcs predisposes to grazing animals development of resistance so requires vigilant Dosing with principal anthelmintic post-treatment FWEC monitoring at predetermined intervals (based on Appropriate for situations where: grazing group is ERP associated with eachdrug class, casually managed (e.g. mult/owner self-care livery seeTable 4.1) premises); young animals grazing; frequent new intake animals Continuous in-feed dosing Daily administration in feed pyrantel Most appropriate if grazing group are mature animals 2.6mg/kg (not licenced in Europe) FWEC, fecalworm egg count; ERR egg reappearance period;epg, eggs per gram: p.o.. oral dosingAlthough stomach bots are non-pathogenic, it is • the class(es) of drug(s) to which anthelminticcommon to incorporate boticidal dosing into the resistance has developed should be indefinitelycontrol programs. The programs designed to con- omitted as a principal anthelmintic from anytrol the major species incidentally control other premises where anthelmintic resistance occurs;minor parasite species. The epidemiology of the andmajor internal parasite species of the horse varies • the efficacy of anthelmintics has beenwith the climate, geographical region, host demo- reported to be markedly less in young than ingraphics, grazing practices and grassland man- adult horses (Herd & Gabel 1990, Herd &agement. As a result, there is no single program for Majewski 1994).parasite control that is applicable to all equine Guidelines for the options for chemical control ofpremises (Proudman & Matthews 2000). equine internal parasites are given in Table 4.3. General points that apply to the use of Although the consensus is that discrete druganthelmintics in parasite control programs are: classes should be used in a slow (annual) rotational• monitoring for anthelmintic resistance should basis in order to delay the selection forbe performed at least annually by either fecal anthelmintic resistance (Herd & Coles 1995), thisegg count reduction tests or in vitro assays issue remains unresolved and contentious. In(Craven et al 1999); field situations where multiple parasite species
  • 71. 4. PARASITICIOES •Table 4.3 Parasiticide regimens In clinical endoparasltlc disease Disease Treatment regimen CommentsCyathostomosis Intensive protocolcombining Moxldectln has the potential for toxicityin ivermectin and benzimidazoles thin, debilitated animals, such that it is Ivermectin, 0.2 mglkg p.o. on probably inappropriate to use in the days 1, 16,31,61 and 91 therapy of clinical cases with cyathostomosis. Fenbendazole 10mglkg p.o. Grazing cohorts of animals with (7.5mglkg in Europe) on days cyathostomosis are likely to harbour 2--6, 17-21, 32-36, 62--66 and immature mucosal cyathostome burdens 92-96 and the owner/manager should be warned that treatment per se has some risk of precipitating overtclinical disease. Forthe cohorts a slightly less-intensive protocol is advised. ivermectin 0.2 mglkg p.o. or moxidectin 0.4mglkg p.o. on days 1, 31, 61 and 91 or fenbendazole 10mg/kg (7.5mglkg in Europe) p.o, on day 2--6, 32-36, 62--66 and 92-96.Largestrongyloid infection Protocol should be directed at both Typically there will be concurrent luminal parasites withinthe intestinal lumen cyathostomes and Immature cyathostome and alsothose migrating within the larval infection such that intensive repeated vasculature. Use parasiticide therapeutic dosingat 10-day ivermectin 0.2 mg/kg p.o., or Intervals may be indicated (see above). moxidectin 0.4mglkg p.o., or When colic is a clinical feature, fenbendazole 10mglkg (Europe administration of anthelmintlcs might 7.5mg/kg) p.o, for 5 consecutive exacerbate signs, and recent anthelmintic days, or fenbendazole 60mg/kg p.o. dosingis a known risk factor for the onset as a single dose of overtcyathostomosis.Ascarid infection Use fenbendazole 10mglkg p.o. for 5 Recommended to repeat 3-4 weeks consecutive days, or afterfirst treatment and avoid further levamisole 8.8mglkg p.o. (not grazing of ascarid egg-contaminated licenced in Europe), or paddocksfor 12 months. ivermectin 0.2mglkg, p.o., or Themode of action results In rapid death moxldectin 0.4 mglkg, p.o. of parasites such that they may cause intestinal obstruction/rupture If given to animals with high burdensof Parascaris equorum.Tapeworm infection Onlypyrantel saltshavelabel claim for good efficacy (> 90%) for the removal of tapeworm Infections. Use pyrantel pamoate 13.2mg/kg p.o. (USA), or pyrantel embonate 38 mglkg p.o. (Europe), or praziquantel1 mglkg p.o.Bot infection Use ivermectin 0.2 mg/kg p.o., or Bots do not cause disease such that organophosphates: trichlorfon prescribing treatment for clinical purposes 40 mg/kg p.o, or dichlorvos is likely, but annual Inclusion of a boticidal 35 mglkg p.o. anthelmintic is often part of a control program (mid-winter dosing)Lungworm infection ivermectin 0.2mg/kg p.o,Strongyloides Use fenbendazole 50mglkg p.o., orwester! infection oxibendazole 10mglkg p.o., or thiabendazole 44mglkg p.o., or Ivermectln 0.2mglkg p.o., or moxldectin 0.4mglkg p.o.Pinworm infection Any anthelmintics at standard dose ratesbut efficacy of piperazine is <70% (continued)
  • 72. 70 EQUINE CLINICAL PHARMACOLOGY Table 4.3 (continued) Disease Treatment regimen Comments Stomach worm Ivermectin 0.2 mg/kg p.o. Gastric lesions unlikely to be treated as infection entity; summer sores treated with ivermectin. Eyeworm infection Physical removal/ophthalmic irrigation; fenbendazole 10 mg/kg p.o, for 5 consecutive days partially effective Onchocerca spp. infection Ivermectin 0.2 mg/kg p.o, May get recurrences because this kills only cutaneous microfilaria and not adult stages, Liver fluke infection Triclabendazole (extra-label use) 15 mg/kg p.o. p.o. oral dosingrequire to be controlled, it is necessary to use more may be repeated in a much shorter timeframe thanthan one drug class during 1 year (Proudman & recommended for control programs.Matthews 2000). In a recent study, it was foundthat 86% of horse owners/managers administered Anthelmintic resistanceeither two or three classes of anthelmintic drug perannum (Lloyd et aI2000). Anthelmintic resistance is an inherited trait that Ultimately, the success of a parasite control develops in response to selection pressure favor-program is best assessed by evidence of a reduced ing survival of those worms that have the inherentincidence or prevalence of parasite-associated genetic ability to survive anthelmintic treatment.disease. However, apart from the general obser- Typically, as the selection pressure is a result ofvation of reduced prevalence of colic following prolonged and/or frequent usage of dewormingS. vulgaris infection (Drudge & Lyons 1986), there doses in parasite control programs (Sangster 1999).is only a single report that provides evidence of Once resistance develops, it appears that rever-the effectiveness of specific equine parasite control sion to susceptibility does not occur (Uhlinger &programs in reducing the incidence of disease Johnstone 1984). To date in equine parasites,(Uhlinger 1990). anthelmintic resistance has only been documented in cyathostomes. There is widespread and well- documented resistance to the benzimidazoles andAnthelmintic regimens for there are also reports of resistance to pyrantel,parasite-associated disease piperazine and phenothiazine (Lyons et al 1999).The guidelines for anthelmintic regimens as a It has been hypothesized that resistance to thecomponent of therapeutic protocols in disease macrocylic lactones will occur within aroundstates are summarized in Table 4.4. These recom- 5 years of the introduction of moxidectin as anmendations are derived from anthelmintic effi- equine parasiticide. This is likely because the per-cacy studies in healthy animals and/or clinical sistent effect of moxidectin (compared with iver-observation, because there are no data from con- mectin) on cyathostomes means that there is atrolled clinical trials. In equine practice, it is not prolonged period of exposure at which drug con-uncommon to make a generic diagnosis of intes- centrations in the host favor the survival of resistanttinal parasitism, in which concurrent treatment worms (Sangster 1999). The same author has sug-with several classes of anthelmintic drug with gested that the first evidence of developing macro-different spectra of activity may be justified (this cyclic lactone resistance will be a reduction in thecontrasts with the general recommendations for egg reappearance period (ERP) following iver-ideal control programs). Also, in disease states, mectin or moxidectin dosing and it is most likelyhigh dosage rates are often appropriate and these to first occur in foals (Sangster 1999). With the
  • 73. 4. PARASITICIDES 71 Table 4.4 Parasiticide regimens In clinical ectoparasitic disease Disease Treatment regimen Notes Lice infestation Piperonyl butoxide: pyrethrum shampoo Skin contact for 10 min then wash out, Permethrin citronella solution repeat after 10-1 4 days Topical sponge or spray, repeat after 14 days Chorioptic mange Doramectin injection 0.3 mg/kg s.c. Repeat after 30 days Fipronil spray applied to affected areas Repeat three times at 5 day intervals Selenium sulfide shampoo Ivermectin injectable applied topically Do not use ivermectin as a pour on as it is to affected areas irritant to equine skin; repeat after 14 days All listed treatments are extra-label use; topical treatments applied after clipping leg hair and general skin hygiene Onchocerciasis Ivermectin 0.2 mg/kg p.o. Repeat after 14 days Moxidectin 0.4 mg/kg p.o. Sarcoptic and psoroptic Ivermectin 0.3 mg/kg p.o. Extra-label; should be effective mange Demodectic mange Extremely rare infection, suggestive of underlying immunosuppressive illness; do not treat with amitraz as it is highly toxic to horses Tick infection Fipronil, direct topical application Extra-label p.O., oral; s.c.. subcutaneousprevalence of anthelmintic resistance recently neural activity by action on the ion exchange asso-reported to be 90% and 30% for fenbendazole and ciated with action potentials. There is rapid develop-pyrantel, respectively, in specific regions (Tarigo- ment of insect muscle contractions, convulsions,Martinie et al 2001), the importance of vigilant paralysis and death. As a class, the syntheticmonitoring for developing anthelmintic resistance pyrethroids have very low mammalian toxicity ascannot be understated. The options for this are absorbed pyrethroids are oxidized rapidly andfecal egg count reduction tests or in vitro assays they are used widely as resid ual insecticides in(Ihler & Bjorn 1996, Craven et al 1999, Pook et al the horse, formulated as solutions or emulsions2002, von Samson-Himmelstjerna et al 2002). and powders. Currently permethrin, cyperme-Recommendations for slowing the spread of thrin, fenvalerate and deltamethrin are indicatedanthelmintic resistance necessitate much greater for use in horses. These are often formulated asveterinary intervention in parasite control pro- combination products containing the synergisticgrams, with considerable horse owner/manager compound piperonyl butoxide; citronella; insectcompliance and improved dissemination of accu- repellent molecules such as the natural pyrethrin,rate information on best practice" parasite control pyrethrum and stabilene (butoxypolypropylenestrategies (Herd & Coles 1995,Reinemeyer 1999). glycols); or a combination of these. The duration of action of pyrethroid products varies from 4 to 14 days depending on the formulation.ECTOPARASITICIDES Miscellaneous ectoparasiticides Macrocyclic lactonesDRUGSPyrethroids The macrocyclic lactones (p. 64) are less effective against ectoparasites in the horse than in rumi-Pyrethroids are contact poisons that enter insects nants, but extra-label use of the injectable solutionthrough their cuticular (skin) surface and disrupt of ivermectin for cattle applied topically to
  • 74. 11 ·eaUINE CLINICAL PHARMACOLOGYchorioptic mange lesions has, anecdotally, some The topical application of insecticides and/ortherapeutic effect. Similarly, extra-label systemic repellents can reduce _exposure. Suggested regi-administration of doramectin appears to have mens include the application of 200ml 0.5% fen-good efficacy in cases of chorioptic mange. valerate alongthe.line of the back. Alternatively 1 litre 0.1% fenvalerate may be applied as a body spray. This should be repeated every 7 days, or fol-Selenium sulfide lowing exposure to rain. Permethrin insecticidalSelenium sulfide (off label) has been reported to repellents are also effective and are used as a pourhave efficacy for the treatment of lice in the horse on preparation (3Q-40ml 4% permethrin). How-(Paterson & Orrell 1995)and it has also been used ever, adverse skin reactions have been reportedin chorioptic mange (Curtis 1999). following application. Cypermethrin and perme- thrin are also used in plastic tags or strips that can be attached to the head collar, mane and tail.Fipronil Repellents may be applied to the horse or toFipronil, a product indicated for the treatment of rugs and hoods. However, this usually has onlyfleas in small animals, has anecdotal reports of limited efficacy.efficacy against equine ticks (Littlewood 1999).The use of fipronil in an individual case of chori- Regimens for ectoparasitic diseaseoptic mange has been reported (Littlewood 2000). Guidelines for specific ectoparasite disease enti- ties are listed in Table 4.4. These are largely basedCONTROL PROGRAMS FOR on clinical observations and include extra-labelECTOPARASITES use of several products.Because of regional differences in the prevalence It is important to apply control measures to allof external parasites, control programs should be horses in a group rather than only treating thosetailored to suit local requirements. In addition to that demonstrate clinical signs of fly, mosquito ortopical and systemic treatments, control should tick problems.aim to minimize exposure to parasites. Generally, the control of lice and mitesinvolves the treatment of both affected individuals MISCELLANEOUS PARASITICIDESand contact animals. Preventative programs arerarely used. However, systemic use of ivermectin Sulfonamides andreduces the extent of egg laying by lice and mites. diaminopyrimidinesWhile ivermectin and moxidectin may have The treatment of EPM is covered extensively insome effect against adult stages, alone they are Chapters 3 and 9. Further information on theseunlikely to treat or prevent infestation effectively antimicrobial agents is presented in Chapter 2.(Littlewood 2000). Prevention of exposure of grazing animals toflies, mosquitoes and ticks is difficult. For control Toltrazurilof flying insects (Diptera spp.), stabling and the The use of toltrazuril and other similar agents inprovision of face masks, hoods and body rugs, horses is covered in Chapter 3.particularly at times of increased parasite activity,can help to reduce exposure. In addition, the Imidocarb diprionateeffect of stabling can be enhanced by using elec-tric fans to induce air movement and by fitting Imidocarb has a direct effect on the structure ofscreens, treated with insecticide, to windows. Babesia spp. and is the drug of choice for equineLimiting grazing to mowed pastures can reduce piroplasmosis (see Ch. 3). Imidocarb has cholin-exposure to ticks. ergic effects and may cause adverse, potentially
  • 75. 4. PARASITICIDES 73fatal effects (colic, ptyalism and diarrhea) in the DiPietro J A, Todd K S, Lock T F et al 1982 Anthelmintic efficacy of ivermectin given intramuscularly in horses.donkey. American Journal of Veterinary Research 43:145-148 DiPietro J A, Hutchens D E, Lock T F et al 1997 Clinical trial of moxidectin oral gel in horses. Veterinary ParasitologyTrypanocidal drugs 72(2):167-177 Drudge J H, Lyons E T 1966 Control of internal parasites ofThere are significant problems with resistance to the horse. Journal of the American Veterinary Medicaltrypanocidal drugs (see Ch. 3). Several formula- Association 148:378-383 Drudge J H, Lyons E T 1986 Large strongyles: recenttions cause significant necrosis at injection sites. advances. Veterinary Clinics of North America 2:263-280In the horse, the most commonly used try- Drudge J H, Szanto T, Wyant A M 1963 Critical tests ofpanocides are sumarin, quinapyramine sulfate thiabendazole as an anthelmintic in the horse. American Journal of Veterinary Research 35:1409-1412and isometamidium, which have efficacy against Drudge J H, Lyons E T, Tolliver S C et al1988 PiperazineT. evansii and T. congolense. Quinapyramine is resistance in population B. equine strongyles: a study ofalso used to treat T. brucei and T. vivax. selection in thoroughbreds in Kentucky from 1966 through 1983. American Journal of Veterinary Research 49:986-994 Duncan J L, McBeath D G, Preston N K 1980 Studies onREFERENCES the efficacy of fenbendazole used in a divided dosage regime against strongyle infection in ponies. EquineAfzal J, Burke A B, Balten P Let al1997 Moxidectin: Veterinary Journal 12:78-80 metabolic fate and blood pharmacokinetics of Duncan J L, Bairden K, Abbott E M 1998 Elimination of 14C-labeled moxidectin in horses. Journal of Agricultural mucosal cyathostome larvae by five daily treatments and Food Chemistry 45:3627-3633 with fenbendazole. Veterinary Record 142:268-271Baggot J D, McKellar Q A 1994 The absorption distribution Elliot M 1973 A photostable pyrethroid. Nature 246: and elimination of anthelmintic drugs: the role of 169-170 pharmacokinetics. Journal of Veterinary Pharmacology Eysker M, Boersema J H, Kooyman F N 1992 The effect of and Therapeutics 17:409-419 ivermectin treatment against inhibited early third stage,Bairden K, Brown S R, McGoldrick J et al 2001 Efficacy of late third stage and fourth stage larvae and adult stages moxidectin 2% gel against naturally acquired strongyle of the cyathostomes in Shetland ponies and infections in horses, with particular reference to larval spontaneous expulsion of these helminths. Veterinary cyathostomes. Veterinary Record 148:138-141 Parasitology 42:295-302Britt D P, Clarkson M J 1988 Experimental chemotherapy in French D D, Klei T M, Foil C S et al1988 Efficacy of horses infected with benzimidazole-resistant small ivermectin in paste and injectable formulations against strongyles. Veterinary Record 123:219-221 microfilariae of Onchocerca cervicalis and resolution ofBritt D P, Preston J M 1985 Efficacy of ivermectin against associated dermatitis in horses. American Journal of Dictyocaulus arnfieldi in ponies. Veterinary Record Veterinary Research 49:1550-1554 116:343-345 Harnett W 1998 The anthelmintic action of praziquantel.Campbell W C, Leaning W H D, Seward R L 1989 Use Parasitology Today 4:144-146 of ivermectin in horses. In Campbell WC (ed) Ivermectin Hennessy D 1997 Modifying the formulation or delivery and abamectin. Springer Verlag, New York, mechanism to increase the activity of anthelmintic pp.245-259 compounds. Veterinary Parasitology 72:367-390Clayton H, Neave R M 1979 Efficacy of mebendazole Herd R P 1986 Epidemiology and control of equine against Dictyocaulus arnfeldi in the donkey. Veterinary strongylosis at Newmarket. Equine Veterinary Journal Record 104:571-572 18:447-452Cornwell R L, Jones R M 1968 Field trials in horses with Herd R P, Coles G C 1995 Slowing the spread of pyrantel tartrate. Veterinary Record 82:586-587 anthelmintic resistant nematodes of horses in UnitedCosta A J, Barbosa 0 F, Moraes FRet al1998 Kingdom. Veterinary Record 136:481-485 Comparative efficacy evaluation of moxidectin gel and Herd R P, Gabel A A 1990 Reduced efficacy of ivermectin paste against internal parasites of equines in anthelmintics in young compared with adult horses. Brazil. Veterinary Parasitology 80:29-36 Equine Veterinary Journal 2:164-169Craven J, Bjorn H, Barnes E H et al1999 A comparison of Herd R P, Majewski G A 1994 Comparison of daily and in vitro tests and a faecal egg count reduction test in monthly pyrantel treatment in yearling thoroughbreds detecting anthelmintic resistance in horse strongyles. and the protective effect of strategic medication of Veterinary Parasitology 85:49-59 mares on their foals. Veterinary Parasitology 55:Curtis C F 1999 Pilot study to investigate the efficiency of 93-104 1 per cent sulphide selenium shampoo in the treatment of Ihler C F. Bjorn H 1996 Use of two in-vitro methods for equine chorioptic mange. Veterinary Record 144:674-675 detection of benzimidazole resistance in equine smallDemeulenaere D, Vercruysse J, Dorny P et al1997 strongyles. Veterinary Parasitology 65:117-125 Comparative studies of ivermectin and moxidectin in the Jacob D E, Huchinson M J, Gibbons L M 1995 Equine control of naturally acquired cyathostome infections in cyathostome infection: suppression of faecal egg output horses. Veterinary Record 141:383-386 with moxidectin. Veterinary Record 137:545
  • 76. 74 EQUINE CLINICAL PHARMACOLOGYJohnson P J, Mrad D R, Shwartz A T et al1999 Presumed Pook J F, Power M L, Sangster N C et al 2002 Evaluation of moxidectin toxicosis in three foals. Journal of American tests for anthelmintic resistance in cyathostomes. Veterinary Association 214:678-680 Veterinary Parasitology 106:331-343Klei T R, Chapman M R, French D D et al1993 Evalution of Prichard R K, Hennessy D R, Steel J W 1978 Prolonged ivermectin at an evaluated dose against encysted equine administration: a new concept for increasing the cyathostome larvae. Veterinary Parasitology 47:99-106 spectrum of effectiveness of anthelmintics. VeterinaryLanusse C, Prichard R 1993 Relationship between Parasitology 4:309-315 pharmacological properties and clinical efficacy of Proudman C J, Edwards G B 1992 Validation of a ruminant anthelmintics. Veterinary Parasitology centrifugation/flotation technique for the diagnosis of 49:123-158 equine cestodiasis. Veterinary Record 131:71-72Larsen M, Nansen P, Grondahl C et al 1996 The capacity of Proudman C J, Matthews J B 2000 Control of intestinal the fungus Duddingtonia f1agrans to prevent strongyle parasites in horses. In Practice 22:90-97 infections in foals on pasture. Parasitology 113:1-6 Proudman C J, French N P, Trees A J 1998 TapewormLittlewood J 1999 Control of ectoparasites in horses. In infection as a significant risk factor for spasmodic colic Practice 21:418-424 and ileal impaction colic in the horse. Equine VeterinaryLittlewood J 2000 Chorioptic mange: successful treatment Journal 30:194-199 of a case with fipronil. Equine Veterinary Education Reinemeyer C R 1999 Current concerns about control 12:144-146 programs in temperate climates. Veterinary ParasitologyLloyd S, Smith J, Connan R M et al 2000 Parasite control 85:163-172 methods used by horse owners: factors predisposing to Ryan W G, Best P J 1985 Efficacy of ivermectin paste the development of anthelmintic resistance in against Strongyloides westeri in foals. Veterinary Record nemotodes. Veterinary Record 146:487-492 117:169-170Love S, Murphy D, Mellor D 1999 Pathogenicity of Sangster N C 1999 Pharmacology of anthelmintic resistance cyathostome infection. Veterinary Parasitology in cyathostomes: will it occur with the avermectin 85:113-122 millbemycins. Veterinary Parasitology 85:189-204Lyons E T, Drudge J H, Tolliver S C et al 1986 Pyrantel Tarigo-Martinie J L, Wyatt A R, Kaplan R M 2001 pamoate: evaluating its activity against equine Prevalence and clinical implications of anthelmintic tapeworms. Veterinary Medicine 81:280-285 resistance in cyathostomes in horses. Journal of theLyons E T, Tolliver S C, Drudge J H 1992 Activity of American Veterinary Medical Association 218:1957-1960 praziquantel against Anoplocephala perfoliata (cestode) Taylor S M, Kenny J 1995 Comparison of moxidectin with in horses. Journal of the Helminthology Society of ivermectin and pyrantel embonate for reduction of faecal Washington 59:1-4 egg count in horses. Veterinary Record 137:516-518Lyons E T, Tolliver S C, Drudge J H 1999 Historical Uhlinger C 1990 Effects of three anthelmintic schedules on perspective of cyathostome: prevalence, treatment and the incidence of colic in horses. Equine Veterinary control programs. Veterinary Parasitology 85:97-112 Journal 22:251-254Martin R J 1997 Modes of action of anthelmintic drugs. Uhlinger C, Johnstone C 1984 Failure to re-establish Veterinary Journal 154:11-34 benzimidazole susceptible populations of smallMcKellar Q A, Benchaoui H A 1996 Avermectins and strongyles after prolonged treatment with non- millbemycins. Journal of Veterinary Pharmacology and benzimidazole drugs. Equine Veterinary Science 4:7-9 Therapeutics 19:331-351 Valdez R A, Dipietro J A, Paul A J et al 1995 ControlledMcKellar Q A, Scott E W 1990 The benzimidazole efficacy study of the bioequivalence of Strongid C® and anthelmintic agents: a review. Journal of Veterinary generic pyrantel tartrate in horses. Veterinary Pharmacology and Therapeutics 13:223-247 Parasitology 60:83-102Mercier P, Chick B, Alves-Branco F et al 2001 Comparative Vandermyde C R, DiPietro J A, Todd K S Jr et al 1987 efficacy, persistent effect, and treatment intervals of Evaluation of fenbendazole for larvacidal effect in anthelmintic pastes in naturally infected horses. experimentally induced Parascaris equorum infections in Veterinary Parasitology 99:29-39 pony foals. Journal of the American Veterinary MedicalMonahan C M, Chapman M R, French D D et al 1995 Association 190:1548-1549 Efficacy of moxidectin oral gel against Onchocerca von Samson-Himmelstjerna G, von Witzendorff C, Sievers cervicalis microfilariae. Journal of Parasitology Get al 2002 Comparative use of faecal egg count 81:117-118 reduction test, egg hatch assay and beta-tubulin codonPaterson S, Orrell S 1995 Treatment of biting lice (Damalinia 200 genotyping in small strongyles (cyathostominae) equi) in 25 horses using 1% selenium sulphide. Equine before and after benzimidazole treatment. Veterinary Veterinary Education 7:304-306 Parasitology 108:227-235Pereira M C, Kohek I Jr, Campos R et al 1991 A field Wall R, Strong L 1987 Environmental consequences of evaluation of anthelmintics for control of cyathostomes treating cattle with the antiparasitic drug ivermectin. of horses in Brazil. Veterinary Parasitology 38:121-129 Nature 327:418-421Perez R, Cabezas I, Garcia M et al1999 Comparison of the Xiao L, Herd R P, Majewski G A 1994 Comparative efficacy pharmacokinetics of moxidectin (Equest®) and of moxidectin and ivermectin against hypobiotic and ivermectin (Eqvalan®) in horses. Journal of Veterinary encysted cyathostomes and other equine parasites. Pharmacology and Therapeutics 22:174-180 Veterinary Parasitology 53:83-90
  • 77. CHAPTER CONTENTSIntroduction 75Pituitary and adrenal glands 75 Drugs acting on the Corticosteroids 75 Equine Cushings disease 76 endocrine systemBrain 80 Endophyte infected fescue toxicosis 80 Janice E. Sojka Michel LevyThyroid gland 80 Laurent CouetilReferences 82 INTRODUCTION The endocrine system consists of not one but multi- ple systems with distinct functions, some of which overlap. Most endocrine systems were originally believed to be simple feedback loops, but this is now viewed as an oversimplification and agents may up- or downregulate hormonal responses at numerous points in a cycle. This chapter will attempt to give an overview of the drugs that affect the various endocrine systems in the horse. The reader must bear in mind that most of what is known about these interactions has been elucidated in other species, primarily in humans. There is a danger in overgeneralizing or applying the find- ings too rigorously to the equine. Unfortunately, until more information is available in horses, the data that are available must be applied. For the sake of clarity, this chapter will be sub- divided into sections. The adrenal and pituitary glands will be described first, followed by the brain and then the thyroid glands. Reproductive endocrinology is a separate topic and as such has been given its own chapter in this book (see Ch. 11). PITUITARY AND ADRENAL GLANDS CORTICOSTEROIDS The class of drug that undoubtedly has the most effect on the various endocrine systems is the cor- ticosteroids (glucocorticoids). Glucocorticoids enjoy a large range of applica- tions including topical ocular therapy, intralesional, 75
  • 78. 78 EQUINE CLINICAL PHARMACOLOGYintraarticular and systemic therapy. The most Excessiveor prolonged administration of exoge-common reason for systemic therapy is the treat- nous corticosteroids has been reported to pro-ment of chronic obstructive pulmonary disease duce adverse effects in horses turned out after(COPD). Other uses include the treatment of a prolonged racing season. This effect is termedimmune-mediated diseases, dermatological con- the "steroid wash-out" or "steroid let down" syn-ditions and other chronic inflammatory diseases. drome. These horses exhibit lack of thrift, depres- Corticosteroids exert their systemic effects on sion, weight loss and a poor hair coat. The etiologythe pituitary-adrenal axis by binding to receptors of this syndrome is hypothesized to be adrenalon the corticotrophs in the pituitary gland and atrophy secondary to prolonged corticosteroidother cells throughout the body. They exert nega- use. However, it has been difficult to reproducetive feedback in this way and suppress the secre- experimentally using the administration of exoge-tion of adrenocorticotropic hormone (ACTH) from nous corticosteroids.the pituitary gland and cortisol from the adrenal Glucocorticoids also have weak mineralocorti-glands. The horse is extremely sensitive to these coid effects as they have some affinity for miner-effects. A single injection of dexamethasone at alocorticoid receptors. The laminitis-producing0.04mg/kg is sufficient to inhibit endogenous effects of the glucocorticoids have recently beencortisol secretion and keep it at extremely low linked to their ability to stimulate mineralocorti-levels for more than 24h. It can take the pitu- coid receptors and produce changes in blooditary-adrenal axis up to 72h to recover from a flow and electrolyte balance.single dose of corticosteroid. In addition, repeated Corticosteroids are also inhibitors of thyroidtopical application or intraarticular injections gland secretion and low thyroid hormone levelshave been implicated in producing systemic cor- may be demonstrable in horses at times whenticosteroid effects in horses. The degree and excessive (exogenous or endogenous) corticos-duration of the suppression of the pituitary- teroid concentrations exist. Older horses withadrenal axis that is produced by depot and longer- equine Cushings disease (ECD) and decreasedacting corticosteroid preparations have not been muscle mass frequently have low thyroid hor-as well documented in horses; however, by and mone concentrations. These animals have euthy-large, the potency of the corticosteroid correlates roid sick syndrome and should not be confusedwith the duration of its suppressive effects. with true hypothyroidism. In these horses, the In an effort to minimize the systemic side- thyroid gland is capable of producing adequateeffects of the corticosteroids, short-acting, less- hormone concentrations and will respond normallypotent forms, including topical or local therapy, when stimulated with either thyroid-stimulatingshould be used whenever possible. Prednisone is hormone (TSH) or thyroid-releasing hormoneshort acting but is poorly absorbed from the gas- (TRH). Thyroid supplementation should not betrointestinal tract and for that reason is of ques- administered to these horses; rather their primarytionable efficacy in the treatment of pulmonary problem should be addressed.and other disorders. It should not be consideredwhen treating severe conditions such as purpurahemorrhagica and advanced COPD, when potent EQUINE CUSHINGS DISEASEcorticosteroid effects are required. Depot forms ofthe corticosteroids are potent but may lead to ECD is nearly always associated with hypertro-an increased incidence of side-effects; conse- phy, adenomatous hyperplasia or, in the mostquently, aqueous formulations, such as dexam- advanced cases, a functional adenoma of the parsethasone, are used most commonly. Some of the intermedia of the pituitary gland. This conditionundesirable side-effects of the systemic corticos- was first described in 1932; however the exactteroids include immunosuppression, iatrogenic pathogenesis, diagnostic plan and appropri-Cushings disease, adrenocortical suppression and ate treatment regimen are still under dispute.laminitis (Lavoie 2003). Although, all breeds of horses and both sexes
  • 79. 5. DRUGS ACTING ON THE ENDOCRINE SYSTEM 77may be affected, there is an increased incidence elevated secondary to exercise, hypoglycemiaof ECD in ponies. and stress. Plasma cortisol levels exhibit a diurnal Functional adenomas of the pars intermedia of rhythm in equines, with morning values higherthe pituitary gland usually occur in aged horses. than evening values. Horses with ECD appear toThe average age at diagnosis is 19-21 years. lose this pattern of variation but this is not diag-However, ECD is being recognized increasingly nostic for ECD since it can also occur in nor-in younger horses. It is no longer unusual to mal horses (Beech 1987). However, a horse withdiagnose ECD in horses aged 8-10 years, which marked diurnal variation in cortisol concentra-exhibit subtle clinical signs of ECD. tions probably does not have ECD. The first sign observed is often abnormal The dexamethasone suppression test is thefat distribution and the owner noticing that the best way to evaluate the pituitary-adrenocorticalhorse or pony is an "easy keeper" that requires axis function in horses. In normal horses, dexam-much less grain or feed than other animals. With ethasone administration in the late afternoon, bytime, other clinical signs also appear. The clinical intramuscular (i.m.) injection at a dose rate ofsigns most often associated with advanced ECD 40 IJog/ kg (approximately 20 mg for a 450 kginclude hirsutism, an abnormal hair shedding horse), depresses cortisol production to less thanpattern, hyperhidrosis, polyuria and polydipsia, lOng/ml (l ug Zdl) by the following morningweight loss and decreased muscle tone. Other and cortisol levels will remain well below base-clinical signs, including infertility, chronic lamini- line for over 24 h. Cortisol levels are usuallytis, lethargy, blindness, seizures, delayed wound slightly depressed after dexamethasone admin-healing, bulging of the supraorbital fat pads and istration to horses with ECD but the degree ofincreased susceptibility to infection may be pres- suppression is less than in normal horses andent alone or with the signs listed above. Laminitis plasma cortisol concentrations rebound morethat is refractory to standard treatment regimens quickly.and infertility are the two signs of ECD recog- The measurement of endogenous ACTH con-nized most commonly in younger horses. These centrations is valuable in the diagnosis of ECDhorses do not usually have abnormally long hair, (Sojka & Levy 1995). Plasma samples for ACTHbut owners often report that these horses shed determination require special handling: bloodtheir winter coats later in the spring than normal. must be collected in chilled disodium ethylene-As these horses grow older, more typical signs of diamine-tetraacetic acid (EDTA) tubes, maintainedECDappear. at 4°C centrifuged promptly and the plasma stored at -70°C until analysis. The ACTH refer- ence range may vary between laboratories. ThereDiagnosis can be an overlap in the values of stressed normalAlthough hirsutism is reported to be pathogno- horses and early cases of ECD.monic for ECD, the diagnosis should be confirmed Horses with ECD have deranged glucoregula-using clinical pathology. Abnormal clinical patho- tory mechanisms and are often insulin resistant.logy results typically occur rather late in ECD and Normal horses show an immediate rise in plasmausually include hyperglycemia, hyperlipidemia glucose concentrations and a return to a baselineand a stress leukogram. level within 1.5 h of intravenous (i.v.)administra- A diagnosis of ECD is usually confirmed by tion of glucose (0.5g/kg). Plasma glucose con-testing the pituitary-adrenal axis. Single or mul- centrations return to baseline with some delay intiple measurements of plasma cortisol are not horses with ECD. Basal insulin levels are alsodiagnostic for ECD. In fact, cortisol concentra- persistently increased, even in the absence oftions vary widely in normal horses and the val- hyperglycemia. When subjected to an exogenousues from horses with pars intermedia dysfunction i.v, insulin tolerance test (0.4IV/kg), horses withare often within the reference range (Dybdal et al ECD do not show a significant decline in blood1994). Additionally, cortisol concentrations may be glucose (Ralston 2002).
  • 80. 78 EQUINE CLINICAL PHARMACOLOGYTreatment the treatment of Parkinsons disease in humans (Beech 1994). It also inhibits the secretion of pro-The most important aspect of keeping horses lactin (and, as such, has been used to inhibit lac-with ECO healthy is excellent basic husbandry tation in humans), produces transient increasesand vigilance for any intercurrent diseases. In in serum growth hormone and decreases inaffected horses that are not persistently hyper- luteinizing hormone concentrations. The actionsglycemic, regular dental care, deworming, foot of pergolide may be decreased if horses are alsocare, responsive nutritional management and given phenothiazine tranquilizers or reserpine.close attention to stable cleanliness and hygiene Controlled studies to establish the pharmaco-are generally all that is needed. Body clipping kinetics (e.g. plasma concentrations, half-life, bio-(with appropriate blanketing) is recommended availability and metabolism) of this drug havefor affected horses that have the characteristic not been performed in horses. This lack of basicheavy hair coat. A combination of hyperhidrosis data has prevented the rational development ofand/ or appropriate sweating means that horses therapeutic plans for the treatment of ECO inwith ECO often end up wet or with heat stress, horses. The current treatment recommendationswhich can lead to many indirect complications are based on information extrapolated from the(Schott 2002). literature in human medicine and clinical dose- Medical intervention is recommended for response trials using limited numbers of horses.horses with ECO that are hyperglycemic, hyper- Pergolide is active and appears to be safe in horseslipemic, infertile or suffering from acute laminitis. with ECO at the doses used currently (Schott et alThere are currently three agents that are recom- 2001). Pergolide is expensive; therefore, it ismended for use in horses with ECO (Table 5.1). recommended that treatment of affected horses is started using a total daily dose of 0.5mg/horse,Pergolide the lowest effective dose reported to date; someTreatment with pergolide, a long-acting, oral ponies can be successfully maintained on 0.25mg/dopamine receptor agonist, is currently the most pony daily. If normal blood glucose concentra-frequently recommended approach. Pergolide is tions are not achieved within 4-6 weeks of start-a semisynthetic ergot alkaloid derivative that is ing therapy, the daily dose can be increased bya potent agonist at both 0 1 and O2 receptors. Its 0.25-0.5 mg/horse. This schedule should bestimulatory effects on postsynaptic dopamine repeated until the dose that maintains the horsereceptors in the nigrostriatal system are used in in euglycemia is reached. Pergolide has been Table 5.1 Comparison of drugs used to treat equine Cushings disease Pergalide Cyproheptadine Bromacriptine Mechanism of action O2 receptor agonist Antiserotonergic, antihistamine O2 receptor agonist Dose 1 Il-g/kg daily: for horses 0.25-0.5 mg/kg daily 0.02 mg/kg twice daily 0.5 mg, daily; for ponies 0.25 mg daily Increase dose over time Yes, by 0.25 mg/day Yes, increase dose or give Not reported if needed same dose twice daily Route Oral Oral Intramuscular Assessment of efficacy Normal blood glucose level, normal Improved appetite, improved As for pergolide endogenous ACTH, dexamethasone hair coat, normal blood suppression test glucose level Precautions None High doses may result in Do not give to pregnant depression or lactating animals ACTH, adrenocorticotropic hormone
  • 81. 5. DRUGS ACTING ON THE ENDOCRINE SYSTEM 79administered safely to pregnant mice and rabbits an appetite stimulant in humans and some of the(Thompson & Montvale 2003). improved clinical condition seen in treated horses Based on both subjective and objective criteria, might be related to an increase in caloric intakemost horses treated to date have shown marked and consequent improvement in the condition ofand ongoing clinical improvement. Pergolide is a the horses. In one pilot study (six horses), weightreplacement therapy; therefore, it must be admin- gain, improved hair coat, a return to or tendencyistered for the lifetime of the horse if it is going to towards normoglycemia and an improved res-have continued therapeutic benefits. In horses ponse to the dexamethasone suppression teststhat are hyperglycemic, blood glucose levels can be were observed. However, these results could notmonitored as a surrogate marker of the response be repeated when horses with ECD were firstto therapy. The reestablishment of euglycemia is stabilized clinically and conditioned to theirevidence of a successful clinical response to ther- new environment (e.g. dewormed, teeth floatedapy. However, establishing euglycemia does not (rasped) and a minimum of 4 weeks at a stablealways coincide with a return to normal pitui- body weight on a high plane of nutrition).tary function. Repeated evaluation of endogenous There is no basic information on the pharma-ACTH concentrations may also be used to deter- cokinetics of cyproheptadine in horses. The rec-mine the response to therapy. If the ACTH levels ommended starting dose extrapolated from dosageare within the reference range, it can be assumed recommendations in humans and corrected forthat the dose of pergolide is adequate. If hyper- metabolic body size is 0.25mg/kg once dailyglycemia is not present at the start of therapy or (Oybdal & Levy 1997).A daily dose of 0.25mg/kgthe return to normal pituitary function is the twice daily can be given if there is no response todesired clinical end-point, a repeat dexamethasone therapy after 6-8 weeks. Higher dose rates pro-suppression test should be carried out to confirm duce lethargy and excess drowsiness in horses.the reestablishment of a functional pituitary- Geldings and stallions may develop paraphimo-adrenal feedback loop. sis during treatment. However, cyproheptadine appears to have a wide margin of safety in horses.Cyproheptadine BromocriptineCyproheptadine is an antihistaminic and antis-erotonergic agent that also has anticholinergic Bromocriptine is an ergot alkaloid dopamineand sedative effects. It is used for its antihista- receptor agonist that activates postsynapticminie actions in humans. It has been used with dopamine receptors. It blocks the release of pro-limited success in the treatment of pars intermedia lactin from the pituitary gland via effects on O2dysfunction in horses. receptors in the tuberoinfundibular system, inhibit- Anecdotal evidence indicates that a positive ing prolactin release and decreasing hyperpro-response is achieved in about 35% of the horses lactinemia and the size of some tumors. It is usedwith ECD treated. Reed reported successful in human medicine in the treatment of prolactin-outcomes in 25 horses treated cyproheptadine secreting tumors and other micro- and macroade-(Reed 1998). Objective studies to elucidate the nomas of the pituitary gland (Anon. 2002). It ismechanism of action of cyproheptadine have also used in the treatment of menstrual disordersfailed to support these clinical findings: no changes and acromegaly and has been recommended aswere identified in the plasma concentrations an adjunct therapy for Parkinsons disease inof proopiomelanocortin-derived peptides or in humans. In the horse, bromocriptine is used muchthe response to dexamethasone administration. less widely for the treatment of ECO than per-Cyproheptadine may have non-pituitary mecha- golide and cyproheptadine.nisms of action, since the horses that respond to There is no basic information on pharmacoki-therapy have normal hair growth and improved netics of bromocriptine in horses. Administrationenergy levels. Cyproheptadine is known to be of O.02mg/kg i.m, twice daily has been reported.
  • 82. 80 EQUINE CLINICAL PHARMACOLOGYBromocriptine is also available as an oral formu- endophytic fungus Acremonium coenophialum. Itlation, but oral administration to horses has not has been proven to be the causative agent of abeen reported. This drug should not be used in host of problems in horses, most commonly prob-pregnant or lactating animals because of its effects lems in pregnant and lactating mares. Theseon prolactin and lactation. Concomitant adminis- include an increased incidence of prolongedtration of bromocriptine and the phenothiazine gestation, stillborn foals, agalactia and thickenedtranquilizers or reserpine is contraindicated. placentas. An increased incidence of laminitis has been reported in younger horses. The hormonalMitotane (a,p-DDD) changes that occur in mares include decreased serum prolactin and progesterone and increasedThe adrenocorticolytic drug mitotane to.p -DOD), serum estrogen concentrations (Cross 1994).used in dogs and humans to treat adrenocorticalhyperplasia, is of no therapeutic benefit in thetreatment of ECD in horses. Treatment The toxin acts as a dopamine receptor agonist inFollow-up to treatment the brain. Dopamine antagonists can reverse the effects of the toxin, as do prolactin stimulators,The response to treatment in horses with ECD is such as TRH and metoclopramide. Treatment isusually judged using the subjective improve- generally accomplished using the O2 receptorment in the clinical signs: improvement or reso- antagonist domperidone. Unlike other O2 recep-lution of the laminitis, reduction in the frequency tor antagonists, domperidone does not cross theof urination, decrease in water intake, shedding blood-brain barrier and, therefore, it does notof hair and weight gain. Repeating a dexametha- produce neuroleptic effects (Evans 2002). Rather,sone suppression test (Williams 1995) or reevalu- it blocks the effect of the fescue toxin on the O2ating endogenous ACTH levels would give an receptors in the pituitary gland, which are out-objective evaluation of the efficacy of treatment. side the blood-brain barrier. Domperidone hasIt is important to follow the overnight test proto- been administered orally (p.o.), at a dose rate ofcol, since full suppression takes an extended 1.1mg/kg once daily, to treat fescue toxicosis inperiod to develop fully and remains low for at pre- and postpartum mares (Evans 2002). Theleast 24h in normal horses (Beech1987). A decrease treated mares remained on the fescue-infectedin plasma cortisol concentrations in horses with pastures but showed none of the typical signs ofECD will not persist for the 17-19 h in which the fescue exposure and went on to foal and lactateovernight protocol is performed. normally. Their prolactin levels were found to be higher than in the untreated control mares and were comparable to normal values. DomperidoneBRAIN also appeared to increase milk production in mares with poor milk production in this study. The phe-Environmental toxins can affect the brain and its nothiazine tranquilizer perphenazine has alsohormone production. Fescue toxicosis is one such been used to increase prolactin levels in agalacticdisease. mares (Cross 1994).ENDOPHYTE INFECTED FESCUETOXICOSIS THYROID GLANDTall fescue is a cool season grass that is grownwidely throughout the world. It has many char- Thyroid disease is one of the most controversialacteristics that make it a valuable forage source. areas in equine medicine. Practitioners run theMuch of the fescue in the USA is infected with the gamut from those who do not "believe in" thyroid
  • 83. 5. DRUGS ACTING ON THE ENDOCRINE SYSTEM 81disease to those who diagnose and treat it fre- In adult horses, thyroid gland enlargement isquently. Many of the clinical signs often attributed usually caused by adenoma of the thyroid gland.to thyroid deficiency are more commonly signs of These tumors are benign and do not, in the vastECO. These include obesity, a thickened neck, majority of instances, alter thyroid function. Theseabnormal fat distribution, infertility and laminitis. masses should be removed surgically if theyIf horses demonstrate these clinical signs without obstruct the trachea or esophagus or if hypothy-ECO, they have recently been characterized as roidism can be documented.suffering from "metabolic syndrome". Equinemetabolic syndrome is analogous to non-insulin-dependent diabetes mellitus in humans. Currently, Diagnosisthere are no recommended medications for Oiagnosis of thyroid disease is made primarilythe treatment of this syndrome. Restricting caloric through the detection of blood thyroxine (T4) andintake and increasing exercise appear to be the triiodothyronine (T3 ) concentrations and throughmost effective means of correcting the syndromes the use of stimulation tests. An assay for endoge-effects. It is noteworthy that experimentally nous equine TSH is not yet available commer-induced thyroid deficiency (removal of the thyroid cially. When (and if) this test becomes availableglands) produces none of the "typical" signs of for use in diagnostic laboratories, it may help tohypothyroidism in horses. Most horses with "thy- shed some more light on thyroid disease in adultroid syndrome" have normal thyroid gland func- horses.tion. A large number actually have normal restingthyroid hormone levels but are treated because Baseline thyroid hormone measurementsthey are "low normal". Others may have restingvalues below the normal range but respond nor- There are a large number of factors that affectmally to stimulation testing. Anecdotal reports resting thyroid hormone levels in adult horses.state that these horses generally appear to respond Thyroid hormones set the resting metabolic rateto thyroid hormone supplementation (lose weight and thus increase at times when the metabolicand regain normal behavior and fertility) despite rate increases and decrease when the metabolichaving no demonstrable thyroid axis problem. It is rate decreases. Euthyroid sick syndrome is theclear that these horses do not have primary thyroid name given to the situation where the thyroiddisease and in most cases have either ECO or gland downregulates in response to debilitatingequine metabolic syndrome. Why they respond to illness or a catabolic state: the body is tryingthyroid supplementation is not known, but the to conserve energy and calories by lowering itseffect may be mediated via alterations in fat meta- basal metabolic rate. In this syndrome, restingbolism produced by the thyroid supplementation. thyroid hormone levels are low although the thy-The term "thyroid hormone responsive syndrome" roid glands are normal. In human patients withbetter describes these horses as it acknowledges euthyroid sick syndrome, thyroid hormone sup-the fact that they respond to treatment, without plementation is associated with increased mor-implying that they have primary hypothyroidism. tality rate because of the primary disease process. Following surgical thyroidectomy, horses It is likely that many horses with ECO have adevelop signs including exercise intolerance, form of euthyroid sick syndrome. Thyroid hor-decreased body temperature and heart rate, mone supplementation is of no help and may beincreased cold sensitivity, increased blood choles- harmful in these animals.terol and thickened facial features. These horses Thyroid hormones are present in the blooddo not become particularly obese or depressed, in two forms; protein-bound and free hormone.or develop laminitis. Low resting thyroid hor- Although the free portion constitutes only approx-mone levels have also been associated with signs imately 1% of the total hormone, it is the metaboli-such as infertility, anhidrosis, myositis, abnormal cally active form and the one that is transportedbehavior, laminitis, agalactia and infertility. into cells to exert its effects. Many drugs have been
  • 84. 82 EQUINE CLINICAL PHARMACOLOGYshown to lower the resting thyroid hormone con- by down regulating their production of T4 andcentrations in humans and other animals. These the normal thyroid axis is maintained. Therefore,include phenylbutazone, glucocorticoids, insulin, when thyroid supplementation is withdrawn, ithalothane, diphenylhydantoin (phenytoin), phe- should be done gradually over a period of sev-nobarbital, sulfonamides, furosemide (frusemide) eral weeks. Horses with low thyroid hormoneand radioopaque contrast (iodine-containing) dyes levels and debilitating disease (euthyroid sick(Sojka1993). In all likelihood, these agents decrease syndrome) should not receive thyroid supple-thyroid hormone levels in horses as well, but only mentation unless a true thyroid deficiency hasthe effects of phenylbutazone (Sojka1993) and glu- been diagnosed definitively.cocorticoids have been documented. Phenylbuta- There is very little published research lookingzone does not displace T4 from the protein binding at blood T4 levels in horses on thyroid supple-sites but does suppress the circulating concen- mentation. A single dose of 20 J-Lg/kg T4 willtrations of both free and total T4 (Sojka 1993). maintain blood levels in the reference range forDespite the large numbers of horses on prolonged 24h (Bayly et al 1996). There are several othertherapy with potentiated sulfonamides (trimetho- forms of T4 available for the treatment of horses.prim/pyrimethamine plus a sulfonamide) for the Iodinated casein, orally at 5.0g/day, is not read-treatment of EPM, side-effects from the action of ily available currently but has been recom-these agents on thyroid hormone concentrations mended in the past (Sojka 1995). Concentratedhave not been reported in horses. Any iodine-con- bovine thyroid extract at a dosage of 10.5g/daytaining compound (such as i.v. radioopaque con- has been reported to return thyroid hormonetrast media) will temporarily inhibit thyroid serum concentrations to normal (Sojka 1995).hormone secretion. In adult animals with normal A horse receiving thyroid supplementationthyroid glands, tolerance will develop and thyroid should have its serum T4 and T3 values moni-hormone levels in the blood will recover to pre- tored every 4-6 weeks and the dosage of the sup-treatment levels even in the face of prolonged plement should be adjusted to maintain serumiodine administration. The fetus and animals with concentrations of thyroid hormone within thedamaged or diseased thyroid glands will not be reference range. If therapy is discontinued, theable to reestablish normal thyroid function in the horse should be weaned slowly from the medica-face of high iodine ingestion and goiter may result. tion to allow its thyroid tissue function time to Bayly and coworkers reported a survey of rest- return to normal.ing thyroid hormone concentrations in racingthoroughbreds (Morris & Garcia 1983). Many of REFERENCESthese healthy animals with no signs of diseasehad resting T4levels significantly below the pub- Anon. 2002 Bromocriptine mesylate. In: Mosbys drug consult. Mosby, St LOUis, MO pp. 111356-111358lished reference ranges for sedentary horses. The Bayly W, Andrea R, Smith J et al 1996 Thyroid hormoneconclusion was that T4 is very susceptible to the concentrations in racing thoroughbreds. Pferdeheikundeeffects of a variety of exogenous stimuli and as 12:534-538 Beech J 1987 Tumors of the pituitary gland (parssuch is a very unreliable indicator on which to intermedia). In: Robinson N E (ed) Current therapy inbase thyroid status in horses. Concentrations of T3 equine medicine, 2nd edn. Saunders, Philadelphia, PA,appeared to be much more stable. Consequently, pp.182-185 Beech J 1994 Treatment of hyophyseal adenomas.if the thyroid status of a horse is to be evaluated Compendium on Continuing Education for the Practicingusing a one-off sample, T3 should be determined. Veterinarian 16:921-923 Cross D L 1994 Effects and remedial therapy associated with the toxins of fescue in gravid mares. In: ProceedingsTreatment of the 40th American Association of Equine Practitioners Annual Convention, Vancouver BC,The effects of thyroid supplementation in horses pp.33-34 Dybdal N 0, Levy M 1997 Pituitary pars intermediawith normal thyroid function are not known. It is dysfunction in the horse Part II. Diagnosis andlikely that the glands respond in most instances treatment. In: Proceedings of the 15th American
  • 85. 5. DRUGS ACTING ON THE ENDOCRINE SYSTEM 83 College of Veterinary Internal Medicine Forum, Schott H C 2002 Pituitary pars intermedia dysfunction: Lake Buena Vista, FL, pp. 470-472 equine Cushings disease. Veterinary Clinics of NorthDybdal N 0, Hargreaves K M, Madigan J E et al 1994 America Equine Practice 18:237-270 Diagnostic testing for pituitary pars intermedia Schott H C, Coursen C L, Eberhart S W et al 2001 dysfunction in horses. Journal of the American The Michigan Cushings project. In: Proceedings of the Veterinary Medical Association 204:627-632 47th American Association of Equine PractitionersEvans T J 2002 Endocrine alterations associated with Annual Convention, San Diego, CA, pp. 22-24 ergopeptine alkaloid exposure during equine pregnancy. Sojka J E 1993 Factors which affect serum T3 and T4 Veterinary Clinics of North America Equine Practice levels in the horse. Equine Practice 15:15-19 18:371-378 Sojka J E 1995 Hypothyroidism in horses. CompendiumLavoie J P 2003 Heaves (recurrent airway obstruction): on Continuing Education for the Practicing Veterinarian practical management of acute episodes and prevention 17:845-851 of exacerbations. In: Robinson N E (ed) Current therapy Sojka J E, Levy M 1995 Evaluation of endocrine function. in equine medicine, 5th edn. Saunders, Philadelphia. PA, Veterinary Clinics of North America Equine Practice pp.417-421 11:415-435Morris D D, Garcia M 1983 Thyroid-stimulating hormone: Sojka J E, Johnson M A, Bottoms G D 1993 Serum response test in healthy horses and effects of triiodothyronine, total thyroxine, and free thyroxine phenylbutazone on equine thyroid hormones. American concentrations in horses. American Journal of Journal of Veterinary Research 44:503-507 Veterinary Research 54:52-55Ralston S L 2002 Insulin and glucose regulation. Medical Economics Staff, PDR Staff and Physicians 2002 Veterinary Clinics of North America Equine Practice Physicians Desk Reference 2003, 57th edn. Thompson 18:295-304 Healthcare, Montrale, NJ. pp. 569-571Reed S M 1998 Pituitary adenomas: equine Cushings Williams P D 1995 Equine Cushings syndrome: disease. In: Reed S M, Bayly W M (eds) Equine retrospective study to twenty four cases and response internal medicine. Saunders, Philadelphia, PA, to medication. In: Proceedings of the 34th British Equine pp.912-916 Veterinary Association Congress, p. 41
  • 86. CHAPTER CONTENTSIntroduction 85Overview of gastrointestinal function 86 Drugs acting on the Motility 86 Drugs affecting alimentary tract motility 88 Secretion and absorption 91 gastrointestinal system Summary 95 Michael J. MurrayEsophagus 95 Drugs used for esophageal relaxation with obstruction 95 Drugs used to treat reflux esophagitis 96Stomach 97 Drugs affecting gastric acidity 99 Drugs acting on gastric mucosal protection 106 Drugs acting on gastric motility 107Small and large intestine 108 INTRODUCTION Pathophysiology of intestinal motility disorders 108 Clinical conditions with intestinal motility Many of the drugs used in horses in veterinary disorders 108 practice affect the gastrointestinal tract. Some of Drugs acting on motility of the small and large these effects are intended and may be beneficial intestine 109 Pathophysiology of intestinal secretory for specific disorders, whereas other effects are sec- disorders 113 ondary to the intended effect and are considered Drugs acting on intestinal secretion and to be adverse effects. This chapter will cover those absorption 113 Agents promoting digestion and absorption 115 drugs that have a direct effect on segments of the Drugs acting on intestinal mucosa 115 gastrointestinal tract and which may, under cer- tain circumstances, alleviate disease and improveAntimicrobial agents 115 gastrointestinal function. The majority of drugsReferences 116 targeting the gastrointestinal tract in equine prac- tice are intended to affect motility of one or more segments of the alimentary tract, to permit heal- ing of injured mucosa or to influence secretion or absorption. In some cases, these drugs are highly specific and effective. For example, omeprazole blocks gastric acid secretion and permits healing of gastric ulcers. In other circumstances, the drug effect is general and variably effective. Metoclo- pramide, for instance, affects the motility of much of the gastrointestinal tract but in many dis- orders/ such as postoperative ileus (POI) and duodenitis-proximal jejunitis, the effectiveness of the drug is highly variable and arguable. Our knowledge of the physiological and patho- physiological mechanisms of the gastrointestinal tract continues to grow rapidly and studies since the mid-1990s have revealed a highly complex interrelationship between the intestinal cells, blood vessels, smooth muscle cells and neurons within bowel segments. The concept of the different layers of the gut (mucosa, lamina propria, submucosa, muscularis, serosa) being anatomically connected 85
  • 87. B8 EQUINE CLINICAL PHARMACOLOGYyet more or less independent is yielding to the extrinsic afferent and efferent fibers interactingrecognition that the cells within the different with several cells, cytokines, neuropeptides andbowel layers "talk" to each other and to cells in growth factors are beginning to be understood.other layers of the intestinal wall via a multitude The ability to manipulate these processes in aof cytokines. The enteric microenvironment is a beneficial manner is in the earliest stages ofdynamic, responsive system and several cell types investigation.should be taken into account when considering In this light, it is unrealistic to expect many ofadaptive changes in the enteric microenviron- the pharmacological agents available currently toment: neurons, enteric glial cells, smooth muscle have their intended effects. Most of these drugscells and the interstitial cells of Cajal, which are act on the most grossly and readily recognizedviewed as the intestinal pacemaker cells (Giaroni elements of gastrointestinal function. Given theet a11999). Mast cells and mesenchymal cells may complexity of normal homeostatic mechanismsalso play an important role in the mediation of and pathophysiological processes, many of ourneuroimmune interactions. These cells and their current drugs must be seen as being rather un-products regulate normal intestinal function and sophisticated. Nonetheless, many traditional drugshelp the gastrointestinal tissue to respond to vari- remain effective tools in our therapeutic arrnen-ous conditions, but they can also initiate and sus- tarium and new drugs have arrived recentlytain pathophysiological events that can severely or are in development for use in gastrointestinalcompromise intestinal function and viability. disorders. Intestinal mucosal cells also can activate medi-ators of inflammation. Intestinal epithelial cellsexpress an array of cytokine receptors and pro- OVERVIEW OF GASTROINTESTINALduce a spectrum of immune mediators, suggest- FUNCTIONing that they play an integral role in mucosalinnate and acquired immunity (Dwinell et a11999). The primary function of the digestive tract is theConsistent with those functions, human intes- digestion and absorption of nutrients. The smalltinal epithelial cells have been shown to upregu- intestine of the horse is presumed to be function-late the expression and secretion of a variety of ally similar to that of other species, albeit havingproinflammatory chemokines in response to infec- adapted to an herbivorous diet that is lower in fattion with enteroinvasive pathogens or stimula- and protein than the diets of carnivorous species.tion with proinflammatory cytokines. Epithelial The equine digestive system is highly adaptedcell-derived chemokines appear to act as media- for the microbial digestion of cellulose and absorp-tors of intercellular communication between the tion of the resulting short-chain fatty acids (SCFA).epithelium and immune and inflammatory cells There are few disorders related to maldigestionin the adjacent and underlying mucosa. or malabsorption in equine species for which The enteric nervous system (ENS) is a fre- there are specific treatments. One example is lac-quent target of pharmacological agents; many are tase deficiency in foals, either innate or acquiredintended to improve propulsive motility. The ENS as a result of enteritis, which can be treated orallyis a highly complex system that not only affects (p.o.) with lactase. Most of the disorders of themotility of the gastrointestinal tract but also is equine alimentary system relate to motility andassociated intimately with cells that can release secretion/ absorption in the different segments ofhomeostatic and proinflammatory cytokines. the gut.Recent evidence suggests possible cross-talkbetween intestinal smooth muscle cells and dor- MOTILITYsal root ganglion cells (Ennes et a11997), a findingthat may affect our understanding of altered Normal intestinal motility is a complex interplayvisceral sensitivity and reinforce the concept of between central and peripheral nerves, local feed-a brain-gut axis. The interactions of intrinsic and back mechanisms and cells in the target tissue
  • 88. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 87that release an array of cytokines which can pro- parasympathetic stimulation promotes motilitymote or inhibit the effects of neural stimulation and secretions in the gut. Parasympathetic stimulion the gut. Alimentary motility has typically been originate in the vagal nucleus and are transmittedstudied by measuring changes in pressure within via the vagus nerve and pelvic nerves. Presynap-the bowel lumen, changes in tension of strain tic fibers synapse in the myenteric and submucosalgauges implanted in the wall, electrical activity plexuses onto postsynaptic fibers that innervate(via electrodes implanted in the serosa) and by intestinal smooth muscle, vessels in the sub-using scintigraphy to assess transit time of radio- mucosa, muscularis mucosa and mucosal cells.labeled materials. The different segments of the Acetylcholine is the neurotransmitter at both theequine alimentary system, esophagus, stomach, synaptic junction and the neuromuscular junc-small intestine, cecum and large colon and small tion. The cholinergic receptors in the plexuses arecolon, have distinct motility characteristics, based nicotinic receptors and on smooth muscle areon their functions. muscarinic M2 receptors. An important electrophysiological feature of the Sympathetic stimuli are generally inhibitory tointestinal tract is the electrical rhythm of the gut, gastrointestinal motility and secretion (De Pontiwith electrical activity coordinated on a rhythm et aI1996). Sympathetic fibers originate in the inter-or pattern of slow waves. It is now thought that mediolateral hom of thoracolumbar segments ofthe interstitial cells of Cajal, which are electrically spinal cord gray matter. Sympathetic pregan-coupled to smooth muscle cells and which prop- glionic fibers synapse onto postganglionic fibersagate slow wave activity, control the rhythmicity in paravertebral ganglia, from which fibers travelof intestinal electrical activity (Der-Silaphet et al to the intestine and terminate on cholinergic fibers1998). The migrating myoelectrical complex that synapse onto smooth muscle. Gastrointestinal(MMC) is a repeating sequence of action poten- receptors of sympathetic fibers include a], a2 andtials in the smooth muscle, which consists of three 132 adrenoceptors, which are inhibitory to gutphases. Phase I is a period of quiescence without motility. Release of norepinephrine (noradrena-action potential activity. During phase II, action line) and binding to a2 adrenoceptors on the cellpotentials are intermittent; this is when most bodies of intestinal cholinergic neurons inhibitspropulsion of ingesta along the intestinal tract these neurons.occurs. During phase III, action potentials are Motilin is a neuropeptide expressed predomi-very active and continuous on slow waves. The nantly in the gastrointestinal tract that stimulatesperiodicity of the MMC in horses is approximately the contraction of gastrointestinal smooth muscle2h (Merritt 1999). throughout the gut (Tonini 1996). Physiologically, The large intestine of the horse is the reser- its most characteristic role seems to be the induc-voir in which microbial fermentation of cellulose tion of coordinated interdigestive antral and duo-occurs. The two primary features of motility in denal contractions (phase III of the migratingthe cecum and large colon are propulsive motility motor complex). The effects of motilin are bothand retrograde motility, which retains ingesta in species and dose dependent. Motilin appears tothe large colon in order to maximize digestive have at least two receptors, one muscular andefficiency (Sellers et al 1982). Electrical activity in the other neuronal. Recent studies in vivo havethe large intestine is characterized by short spike emphasized the importance of the latter pathwaybursts and long spike bursts, with the long spike and it is currently hypothesized that motilin actsbursts propagated aborally and associated with on neurons in the myenteric plexus to releasecontractions of the wall of the colon (Roger et al acetylcholine and other excitatory neurotrans-1985). mitters. Components of neuromuscular stimuli to the The NANC neurotransmitter system includesgut include parasympathetic, sympathetic, moti- several substances that may be inhibitory orlide and non-adrenergic, non-cholinergic (NANC) excitatory. Inhibitory NANC neurotransmittersstimuli and receptors (Merritt 1999). In general, include adenosine trisphosphate (ATP), vasoactive
  • 89. 88 EQUINE CLINICAL PHARMACOLOGYintestinal peptide and nitric oxide (NO). These (Katschinski et aI1995). High doses of bethanecholneurotransmitters mediate descending inhibition increase release of gastrin, cholecystokinin andduring peristalsis and receptive relaxation. Sub- pancreatic polypeptide. Unlike in other species,stance P is an excitatory NANC transmitter bethanechol does not increase gastric acid outputinvolved in large intestinal contraction through in horses (Sandin et al 2000), but it does causeactivation of neurokinin 1 receptors. Recent stud- a small but significant increase in gastric fluidies have provided evidence that inducible NO may collection in cannulated horses (Thompson et alcontribute to ileus in various diseases and that 1994), perhaps reflecting non-parietal secretionsinhibitors of NO synthesis may have a role in the derived from the pancreas.future in the treatment of ileus (Kalff et al 2000). Bethanechol has been shown to increase gas- tric contractility and hasten the emptying of liquidDRUGS AFFECTING ALIMENTARY and solid-phase radiolabeled materials from theTRACT MOTILITY stomach of normal horses (Ringger et aI1996). The effects of bethanechol in horses are not restrictedSeveral drugs are used for alimentary tract motil- to the upper alimentary tract, because bethane-ity disorders in humans and there are excellent chol also significantly enhances the rate of cecalreviews on their mechanisms of action and clini- emptying of radiolabeled markers in normalcal use (Longo & Vemava 1993, McCallum 1999, ponies (Lester et aI1998a).Tonini 1996). Bethanechol can produce cholinergic side- effects, such as abdominal discomfort, sweating and salivation, but these are seen either when theCholinomimetics recommended dose rates (0.02mg/kg subcuta-The cholinomimetics act on the neuromuscular neous (s.c.) three times a day, or 0.35mg/kg p.o.junction (muscarinic), either by directly stimulat- three to four times a day) are exceeded or whening the acetylcholine receptors on the smooth the drug is administered intravenously (i.v.), Side-muscle cells or by increasing the levels of acetyl- effects occur infrequently in horses when the drugcholine in the neuromuscular junction by block- is administered at the recommended dose rate.ing cholinesterase activity. Some cholinomimetics,such as bethanechol, can act both at the level ofthe myenteric plexus (nicotinic) and directly on Benzamidesthe intestinal smooth cells. The primary benzamides used to enhance gastro- intestinal motility are metoclopramide and cisa-Bethanechol pride. Each drug has its benefits and hazards, and recently cisapride was withdrawn from theBethanechol is a synthetic derivative of acetyl- market because of its potential to cause fatalcholine and is not degraded by cholinesterase. cardiac arrhythmias in humans.Bethanechol has been used in humans to increaselower esophageal sphincter (LES) tone and to Metoclopramideenhance gastric emptying. The cholinergic side-effects (salivation, abdominal discomfort) of betha- Metoclopramide is a first-generation benzamide.nechol have reduced its use in human medicine The drug acts presynaptically, mainly as asince the introduction of metoclopramide and 5-hydroxytryptamine (5-HT; serotonin) 5-HT4cisapride. receptor agonist and 5-HT3 receptor antagonist but Bethanechol has been shown to stimulate it is also an antagonist at dopamine 1 (01) and 2antroduodenal motility in humans and laboratory (0 2) receptors (MacDonald 1991). The net effect ofanimals. In one study in humans, bethanechol the interactions with these receptors is to facilitatecaused a dose-dependent stimulation of antro- acetylcholine release from enteric neurons and pro-duodenal motility and gastropancreatic secretion mote smooth muscle contraction. Metocloprarnide
  • 90. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 89is used in humans to increase LES tone, improve of stimulation of the O2 receptors is diminishedgastric emptying and minimize the nausea that release of acetylcholine at the neuromuscularmay accompany several conditions. junction. The effects of stimulation of dopamine receptors in the alimentary tract include reductionCisapride of LES tone, reduction of gastric tone and intra- gastric pressure and inhibition of antroduodenalCisapride is a second-generation substituted ben- coordination.zamide that is used to treat a variety of conditionsin humans, including gastroesophageal refluxdisease, peptic ulcer disease, intestinal pseudo- Metoclopramideobstruction and constipation (Washabau & Hall Metoclopramide has inhibitory effects on dopa-1995).The drug appears to act as a 5-HT4 agonist, mine 0 1 and O2 receptors, which influences itswhich enhances release of acetylcholine from prokinetic activity and CNS side effects.neurons in the myenteric plexus. In contrast tometoclopramide, cisapride does not have anti-dopaminergic effects. The density and distribution Domperidoneof 5-HT4 receptors varies along the gastrointesti- Oomperidone is a competitive antagonist atnal tract and between species; therefore, extrapo- peripheral O2 receptors. It influences the motilitylation of effects between species should be done of gastric and small intestinal smooth muscle andwith caution. Cisapride has the advantage over has been shown to have some effects on the motormetoclopramide in that it affects colon motility function of the esophagus (Barone 1999). It alsoand induces defecation in constipated human has antiemetic activity as a result of blockade ofpatients. In addition, it has no central nervous dopamine receptors in the chemoreceptor triggersystem (CNS) effects because it does not affect zone. In some controlled clinical trials, domperi-dopaminergic receptors. done provided better relief of symptoms (anorexia, There are reports that suggest that cisapride is nausea, vomiting, abdominal pain, early satiety,efficacious in the management of intestinal dis- bloating, distension) than placebo in humaneases in horses, including persistent large colon patients with symptoms of diabetic gastropathy.impaction, equine grass sickness and to prevent However, domperidone has not appeared to offerPOI after small intestinal surgery. Cisapride is any advantages over other prokinetic drugs. Veryabsorbed very poorly when administered p.o. little domperidone crosses the blood-brain bar-(Steel et a11998) and is absorbed erratically when rier; therefore, reports of eNS adverse effects inadministered rectally to horses (Cook et aI1997). humans are rare.This limited its use in equine practice in the USA Oomperidone has recently received attentionsince the drug was available in tablet form only. as a therapeutic agent for agalactia in mares graz-However, cisapride was withdrawn from the ing endophyte-infected fescue, principally throughmarket in the USA and Europe in 2000 because of its role in enhancing prolactin release (Redmondinfrequent adverse cardiac side-effects in humans, et al 1994). The dose of domperidone recom-including lengthening of the QT interval and the mended for fescue endophyte-associated agalactiadevelopment of a potentially fatal arrhythmia, in mares (1.1 mg/kg daily) is considerably greatertorsades de pointes. than that used to treat motility disorders in humans (0.2--0.3 mg/kg). Effects on gastrointestinal motil- ity (increased borborygmi, increased defecation)Dopamine antagonists are not apparent in mares given domperidone toTwo sets of dopamine receptor are present in the increase milk production. The potential prokineticgut: 0 1 receptors are located on effector cells and effects of domperidone have not been studiedO 2 receptors predominate on the cell bodies in extensively in horses; however, Gerring & Kingthe myenteric and submucosal plexuses. The result (1989) reported modest efficacy of domperidone
  • 91. 90 EQUINE CLINICAL PHARMACOLOGY(0.2mg/kg i.v.) in an experimental model of ileus gastrointestinal motility. Erythromycin has pro-in two ponies. The drug is commercially available kinetic activity in most species studied, includingin Europe but not in the USA, where it is avail- horses. Erythromycin probably exerts its proki-able through some compounding pharmacies. netic effects by enhancing the release of motilin through cholinergic and serotoninergic mecha- nisms and by direct interaction with motilin recep-Aminoguanidine indoles tors (Tonini 1996). Erythromycin binds to motilin receptors readily. There are regional differencesUnlike metoclopramide and cisapride, the amino- in response to erythromycin within the intestineguanidine indoles have purely 5-HT receptor ago- and differences between species, which may cor-nist effects. The molecules are structurally similar respond to differences in motilin-receptor density.to 5-HT,with different substitutions affecting recep- Depending on the dose administered, erythromy-tor affinity and specificity. cin has many effects on motility, including initiat- ing new MMC complexes, increasing gastric fundicTegaserod tone and increasing antroduodenal motility. Other macrolide antimicrobials, such as clar-Tegaserod maleate is a partial agonist that binds ithromycin and azithromycin, appear to interactwith high affinity at human 5-HT4 receptors with motilin receptors. Current investigationsbut has no appreciable affinity for 5-HT 3 or center on developing motilides that lack antimi-dopamine receptors. It has moderate affinity for crobial activity, thus not disrupting the normal5-HT 1 receptors. Tegaserod, by acting as an ago- intestinal flora, but have more potent effects onnist at neuronalf-Hf, receptors, triggers the release motility. Isolation and purification of motilin recep-of further neurotransmitters, such as calcitonin tors will facilitate the development of drugs thatgene-related peptide from sensory neurons. The have highly specific motilide activity.activation of 5-HT4 receptors in the gastrointesti-nal tract stimulates the peristaltic reflex andintestinal secretion, as well as inhibits visceralsensitivity. The drug has been shown to increase Narcotic agonists and antagonistsemptying of the stomach, small intestine and large The narcotic drugs affect the gastrointestinal tractintestine in animals and humans (Degen et al2001, by alleviating pain perception, altering motilityNguyen et al 1997). An interesting feature of the and affecting secretion. Narcotics interact with thedrug is its ability to reduce the sensation of col- intestinal tract via opioid f.L and K receptors, whichorectal distension (Camilleri 2001), which may have opposing effects, depressing and promo-have applications in horses. Tegaserod maleate is ting propulsive motility, respectively (De Luca &licensed in Europe, Canada and the USA for use Coupar 1996). The presence of these receptors inin women with irritable bowel syndrome. In one the gut implies a functional role for opioids inreport, tegaserod maleate was given to four horses intestinal function. A recent study found thatat a dose rate of 0.2mg/kg twice daily for four slowing of intestinal transit by fat in the distaldoses (Lippold et aI2002). There was a significant half of the gut depends on an opioid pathwayincrease in the transit rate of barium-filled spheres located in the efferent limb of this response (Zhaogiven by nasogastric tube at the time of adminis- et al 2000). Additionally, opioid K receptor ago-tration and an increase in fecal output. nists that act peripherally have been shown to inhibit spinal cord pathways of visceral nocicep- tion (Ness 1999) and such drugs may be useful inMotilides treating painful abdominal disorders.The motilides interact with motilin receptors on Narcotics are no longer typically used in thegastrointestinal neurons and smooth muscle, management of gastrointestinal disorders instimulating smooth muscle contraction and horses because they are controlled substances and
  • 92. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 91other drugs can effectively provide visceral anal- chloride secretion and decrease sodium and chlo-gesia. Likewise, although the narcotic antagonist ride absorption. Conversely, catecholamines tendnaloxone was shown to increase colonic motor to promote increased sodium and chloride absorp-activity in experimental ponies (Roger et al1985), tion in the gut, via a2 adrenoceptors. The effectsinhibition of f..l. receptors by naloxone would be of these stimuli are dose dependent and can varyexpected to augment the horses perception of between different segments of intestine. Increasedvisceral pain (Kamerling et aI1990). Advances in levels of prostaglandins E] and E2 promote intes-targeting specific K receptors may result in drugs tinal secretion, which is mediated through cyclicthat can both improve motility and provide vis- nucleotides and intracellular calcium metabolism.ceral pain relief. Other modulators of intestinal secretion and absorption of fluid and electrolytes include VIP, opioids and NO.SECRETION AND ABSORPTION There is active sodium absorption throughoutThe different segments of the alimentary tract have the intestine that is powered by cellular energyspecialized secretory and absorptive functions. derived from the sodium pumps (Na+,K+-ATPase)The initial secretion of the equine alimentary in the basolateral membrane of intestinal epithe-system is saliva, which is produced in copious lial cells. This energy powers the exit of sodiumamounts when horses eat. The equine esophagus from the cell against an electrochemical gradient.has no secretory or absorptive function, although The movement of virtually all other solutes againstin other species submucosal glands secrete mucins their electrochemical gradients occurs by trans-and a variety of polypeptides such as growth port processes that are "secondarily active" infactors. The primary secretory product of the that they are coupled, either directly or indirectly,stomach is aqueous hydrochloric acid; basal to the movement of sodium ions. Sodium entrygastric secretions approximately 100- 200 f..l.Eq/h into the enterocyte is energetically downhill andper kg for hydrochloric acid and 11/h for water is believed to involve three major mechanisms: (i)(Kitchen et al 1998a). The stomach probably electrodiffusion, via selective sodium channels;has minimal absorptive functions. There is a (ii) electroneutral sodium chloride absorption vialarge non-parietal (non-acidic) component of the "coupled" cation (Na+-H+) and anion (Cl"-secretion, which has been measured in secretions HC0 3- ) exchange; and (iii) electrogenic solute-obtained from the stomachs of cannulated linked sodium absorption, in which sodium ishorses; this is thought to come from the duode- transported into enterocytes coupled to the absorp-num (Kitchen et al 1998b). These secretions have tion of organic solutes, including glucose, aminoa high pH, high sodium content, are stimulated acids, bile salts, water-soluble vitamins and organicby pentagastrin infusion and probably originate acids.from the pancreas. The movement of water and electrolytes across The duodenum and jejunum are capable of both the mucosa in the cecum and large intestine isabsorption and secretion but absorption usually linked to the intraluminal production of SCFAs,predominates. Regulation of intestinal secretion primarily acetic acid, and their absorption acrossand absorption is highly complex and involves the mucosa (Argenzio & Stevens 1975).On a dailyextrinsic and intrinsic neural stimuli, numerous basis, there is normally net absorption of water andreceptor types and intercellular and intracellular electrolytes in the cecum and large colon. How-transport pathways. Intracellular pathways of ever, there are frequent large fluxes of water andelectrolyte transport involve membrane-associated electrolytes into the lumen of the colon, particu-receptors that activate cyclic nucleotide metabo- larly after meal feeding, in horses (Clarke et allism, membrane calcium channels and intracellular 1990). Equine feeding regimens that entail thecalcium metabolism, luminal and basal chloride twice daily feeding of concentrates are associatedchannels and multiple sodium transport channels. with episodes of secretion of large volumes ofCholinergic stimuli tend to stimulate intestinal fluid into the intestine, resulting in transient
  • 93. 92 EQUINE CLINICAL PHARMACOLOGYhypovolemia (up to 15% loss of plasma volume) heat-labile toxin and Salmonella heat-labile toxin.(Clarke et al 1990). Initially, with abundant sub- E. coli heat-stable toxin, which is responsible forstrate for fermentation, the concentration of most cases of travelers diarrhea and has beenSCFAs increases, which results in a net flux of reported in diarrheic foals (Holland et al 1989),fluid into the lumen. This fluid tends to neutral- induces cGMP. Prostaglandins E1 and E2 have aize the increase in osmolality created by the role in intestinal fluid secretion and these media-SCFAs. As SCFAs are absorbed into the mucosa tors have been shown to be associated with bothand their concentration in the lumen decreases, active and passive secretory processes (Eberhart &water is reabsorbed into the colonic mucosa. The Dubois 1995).effects of these large fluid fluxes on the develop- The role of NO in intestinal fluid and electrolytement of large intestinal disorders remains to be secretion depends upon whether the conditionsfully characterized, but they contrast sharply with under study are physiological or pathophysio-the more modest intestinal fluid fluxes that occur logical (Izzo et al 1998). In physiological condi-when horses are fed small meals frequently. Pre- tions, constitutive NO seems to promote fluidsumably, the large fluid fluxes that accompany absorption, based on the findings that NO syn-meal feeding in horses are more susceptible to per- thase inhibitors convert net fluid absorption toturbation and disruption of digestive processes. net secretion in several animal species. This pro- Passive secretion of water into the lumen of the absorptive mode involves the ENS, the suppres-intestine depends on the semipermeable diffusion sion of prostaglandin formation and the openingcharacteristics of the mucosa of the intestine and of basolateral potassium channels. However, inthe osmotic and oncotic differences exerted across some pathophysiological states, NO synthase maythis semipermeable barrier. The permeability of be produced at higher concentrations and is capa-capillaries in the submucosa and mucosa and ble of evoking net secretion. NO synthase con-the density of intercellular tight junctions in the tributes to the diarrheal response to the laxativemucosa determine the permeability of the mucosa. action in the rat of several intestinal secretogogues,The density of tight junctions is very high in the including castor oil, phenolphthalein, bisacodyl,stomach, relatively low in the small intestine and magnesium sulfate, bile salts and cascara.moderate in the large intestine and small colon. In diseases of the small intestine, active secre-tion caused by cyclic nucleotide stimulation can Drugs affecting gastrointestinalresult in a large volume of water and electrolytes secretionmoving into the lumen. Additionally, enteric neu-ron activation of mast cells can increase intestinal Few drugs truly decrease the secretion of elec-capillary permeability and promote passive fluid trolytes and water from the intestine and the usesecretion. Diseases that increase intestinal perme- of antisecretory drugs to treat acute diarrhea inability can result in passive secretion of protein- humans has limitations. Also, some drugs thatrich fluid into the intestinal lumen. Active secretion have appeared to have antisecretory effects inof electrolytes and water is a feature of many experimental animal models have not provendiarrheal disorders and can be stimulated by bac- widely useful clinically in humans or horses. Forterial enterotoxins. Several bacterial enterotoxins instance, the (l2 adrenoceptor agonist drugs cloni-interact with intestinal epithelial cell membrane dine and lidamidine have demonstrable antise-adenylate cyclase or guanylate cyclase, resulting cretory effects in several animal species, butin increased cAMP or cGMP. These, in tum, acti- related drugs, such as xylazine and detomidine,vate basolateral chloride channels, resulting in do not appear to share these effects in horses. Thean increase in the luminal secretion of chloride, non-steroidal anti-inflammatory drug (NSAID)accompanied by sodium and followed by water indomethacin has shown apparent antisecretory(Gemmell 1984). Bacterial enterotoxins that stim- effects in a variety of models of intestinal fluidulate cAMP include cholera toxin, Escherichia coli secretion, including those involving enterotoxin
  • 94. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 93and live enteropathogens. Similar antisecretory Somatostatin analogseffects of NSAIDs, such as flunixin meglumine, Cells throughout the gastrointestinal tract releasehave not been apparent in a clinical setting, despite somatostatin. Somatostatin inhibits acid secre-the widespread use of NSAIDs in horses with tion in the stomach and it promotes absorption ofdiarrhea. sodium, chloride and water in the small intestine and colon (Krejs 1986). The somatostatin analogs octreotide and lanreotide have been shown toLoperamide decrease intestinal secretion in animal modelsLoperamide is a narcotic agonist acting on opioid (Botella et al 1993) and in humans with specificf.1 receptors. Loperamide has mild antisecretory metabolic intestinal secretory disorders; how-effects but its primary effects are to increase seg- ever, these drugs are not used widely in humanmental intestinal contractions and slow the medicine. In one study in horses, octreotide waspropulsion of intestinal contents (Ruppin 1987). shown to decrease gastric acidity (Sojka et alLoperamide has been associated with reduction 1992) but its effects on intestinal or colonicin diarrhea in many clinical studies in humans secretion in horses have not been reported.but has minimal impact on the patients fluid andelectrolyte balance because the secreted fluidremains in the bowel lumen. Loperamide can have Oral replacement solutionsdose-dependent CNS depressant effects. Loper- Oral replacement solutions (ORSs) are used exten-amide also has been shown to promote intestinal sively in humans with acute diarrhea, character-bacterial overgrowth because of promoting the ized by large losses of water and electrolytes,retention of fluid within bowel segments (Duval- caused in many cases by enterotoxigenic bacteria.Iflah et al 1999), which may favor the prolifera- Oral replacement therapy is indicated in manytion of enteropathogenic bacteria. cases of equine diarrhea, particularly in foals. An effective ORS should include carbohydrate as an energy source, sodium, chloride, potassium ionsRacecadotril and a base (bicarbonate or citrate); the concen-Enkephalins are pentapeptides that bind to opi- trations of these constituents should be such thatate receptors. In the gut, enkephalins promote the solution is somewhat hypotonic (Table 6.1)the absorption of sodium, chloride and water (Thillainayagam et al 1998). The standard ORS(Dobbins et al 1980). Racecadotril is an oral recommended by the World Health Organizationenkephalinase inhibitor used in France and the (WHO) contains 90 mEq/1 (1 mmol/l) sodium,Philippines for the treatment of acute diarrhea. It 20mEq/1 (1mmo1/1) potassium, 80mEq/1prevents the degradation of endogenous opioids (1rnmol/I) chloride and uses glucose as the car-(enkephalins) and thus promotes absorption bohydrate source and bicarbonate as the baseof water and electrolytes from the intestinal source. Although standard glucose-electrolytelumen (Matheson & Noble 2000). Studies have ORSs have proved highly effective in achievingdemonstrated the efficacy of racecadotril in two and maintaining rehydration, it does not reducemodels of hypersecretory diarrhea: infusion of stool volume or the duration of the diarrheal ill-cholera toxin and castor oil induced diarrhea. ness. Recent ORS formulations used in humansMoreover, unlike loperamide, racecadotril did not only replace lost water and electrolytes butnot prolong transit time in the small intestine may also reduce intestinal secretions. Incorporat-or colon. Further experiments have shown that ing rice components into the ORS appears toracecadotril does not promote bacterial over- reduce stool volume and recently a componentgrowth in the small intestine (Duval-Iflah et al of rice was shown to decrease cAMP-mediated1999). There are no reports on the use of intestinal secretion (Mathews et al 1999). Theseracecadotril in horses. approaches to ORS therapy may have utility in
  • 95. 94 EQUINE CLINICAL PHARMACOLOGY Table 6.1 Commercially available oral rehydrating solutions Nat K CI- Carbohydrate Base source Base %w/v (mEq/l) (mEq/l) (mEq/l) source (mEq/l) Rehydrating solutions Rehydralite 75 20 65 Glucose Citrate 30 2.5 WHO formulation 90 20 80 Glucose Bicarbonate 30 2 Maintenance fluids Resol 50 20 50 Glucose Citrate 34 2 Pedialyte 45 20 35 Glucose Citrate 30 2.5 Enfalyte 50 20 40 Rice syrup solids Citrate 34 3 WHO. World Health Organizationequine patients, although introduction of fer- colonic bacterial flora growth in vitro. In one studymentable carbohydrates into the adult equine in humans with histological colitis and chronicdigestive tract should be done with caution. diarrhea, BSS significantly improved fecal consis- tency, decreased frequency of defecation and fecal weight in 85% of patients and resolved histological colitis in 75% of patients. After an 8 week courseDrugs acting on intestinal mucosa of treatment, clinical remission of diarrhea lastedBismuth subsalicylate for as long as 28 months without further treat- ment (Fine & Lee 1998).Bismuth subsalicylate (BSS) is used widely in In humans, the absorption of the heavy metalhumans for treatment of diarrhea and is specifi- bismuth is negligible «1%) but salicylate iscally recommended for the prevention of travelers absorbed readily (Nwokolo et al 1990), with asdiarrhea. The precise mechanism of action remains much as 95% of the administered dose excretedundetermined and although the end result of treat- in urine. No data are available on the degree andment with BSS is reduction in diarrhea (Figueroa- consequence of absorption of bismuth or salicylateQuintanilla et al 1993), its effect is probably not in horses.related to a direct antisecretory mechanism. Sali-cylates have been shown to stimulate intestinalfluid and electrolyte absorption per se but it is Prostaglandin analogslikely that, in many cases of colitis, resolution ofinflammation and restoration of a normal surface Damage to the intestinal mucosa is a well-epithelium is required to restore mucosal function. characterized adverse effect of excessive admin-When used as pretreatment or coadministered, istration of NSAIDs. Mucosal changes includeBSS significantly reduced the fluid secretory mucosal atrophy, erosion and ulceration through-response to E. coli LT enterotoxin and cholera toxin out the alimentary tract. The prostaglandin E1in intestinal loops of rabbits and pigs (Ericsson et al analog misoprostol is used to prevent and treat1990). However, when administered even 5 min gastrointestinal mucosal injury in humans takingafter the enterotoxins, BSS had no significant effect NSAIDs on a chronic basis (Schoenfeld et alon enterotoxin-stimulated intestinal secretion. 1999). In one report, the coadministration ofThese results suggest that BSS adsorbs or neutral- phenylbutazone with a prostaglandin E2 analogizes bacterial enterotoxins but does not alter the significantly attenuated the alimentary mucosaleffect of enterotoxins once they have bound to injury associated with the administration ofintestinal mucosa. BSS also modulates normal phenylbutazone alone (Collins & Tyler 1985).
  • 96. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 86SUMMARY DRUGS USED FOR ESOPHAGEAL RELAXATION WITH OBSTRUCTIONIn the following sections the specific alimen-tary tract segments are described together Adrenergic agonist drugswith drugs having specific effects on these The Cl2 adrenergic receptor agonists xylazine andsegments. detomidine are used frequently to relax the eso- phagus in horses with esophageal obstruction. No studies document the utility of this class of drugs in relieving esophageal obstruction per se,ESOPHAGUS although their sedative properties permit the vet- erinarian to attempt to relieve the obstruction.Most disorders requiring pharmacological inter- Also, a horse sedated with an Cl2 adrenergic recep-vention in foals and horses involve obstruction to tor agonist will drop its head below the level of the passage of ingesta or damage to the stratified the thorax, which will reduce the opportunity forsquamous mucosal lining of the esophagus sec- aspiration of saliva and ingesta. There is indirectondary to gastroesophageal reflux or motility dis- evidence that the Cl2 adrenoreceptor agonists causeorders. In the proximal two-thirds of the equine esophageal relaxation and may facilitate relief ofesophagus, the tunica muscularis comprises stri- esophageal obstruction. Detomidine was shownated skeletal muscle, whereas in the distal third to alter esophageal transit of barium contrast, the muscular layer is smooth muscle. The upper including dose-dependent increases in the transitesophageal sphincter prevents esophagopha- time, the retention of barium within the longitu-ryngeal reflux, with subsequent tracheobronchial dinal mucosal folds and retrograde peristalsisaspiration and hindrance of air distention of and pooling of contrast agent within the esopha-the esophagus during inspiration. The LES is gus at both the thoracic inlet and caudal to thefunctional smooth muscle located at the gastro- base of the heart (Watson & Sullivan 1991).esophageal junction. The LES restricts gastro- Doses of xylazine and detomidine used in theesophageal reflux and permits the passage of treatment of esophageal obstruction should be suf-ingested material from the esophagus to the ficient to permit the veterinarian to pass a naso-stomach during relaxation. Normally, the LES gastric tube or endoscope and attempt to relieveremains closed owing to intrinsic myogenic tone the obstruction while causing minimal traumaand gastric distention with ingesta. to the horse. Xylazine may be preferable during Motor innervation to the striated muscle of the initial examination because the obstructionthe esophagus includes the pharyngeal and may have resolved spontaneously or may be eas-esophageal branches of the vagus nerve, which ily relieved by the veterinarian. With more diffi-originate in the nucleus ambiguus of the medulla cult obstructions, detomidine may be preferable(Richards & Sugarbaker 1995). Esophageal peri- because of its longer duration of action (up to 45-stalsis and sphincter function are controlled by 60min), which is desirable to limit aspiration ofthe autonomic nervous system, with contribu- saliva and ingesta. Xylazine can be administeredtions from parasympathetic, sympathetic and i.v, at 0.5-0.7 mg/kg and, for more prolongedenteric divisions. Proximal portions of the esoph- sedation, detomidine can be administered i.v, oragus, including the upper esophageal sphincter, intramuscularly (i.m.) at 0.02 mg/kg.are under direct (cholinergic) control of vagal The 132 adrenergic agonists are used for bron-motor neurons located in the nucleus ambiguus. chodilatory effects in horses and may facilitateIntramural enteric nerves, with a contribution esophageal relaxation. In humans, peristaltic ampli-from vagal preganglionic fibers arising in the tude was increased in the distal smooth muscledorsal motor nucleus of the vagus control the dis- part of the esophageal body after infusion of thetal esophageal regions, including the LES. non-selective beta blocker propranolol (Lyrenas
  • 97. 96 EQUINE CLINICAL PHARMACOLOGY1985). After infusion of the 132 adrenoceptor horses. The disorder is seen most often in foals andagonist terbutaline, a profound decrease in it occurs as a result of gastric outflow impairment.esophageal peristaltic amplitude was reported. Mucosal injury can range from mild erosions toPeristaltic velocity in the distal part of the esoph- severe ulceration. Severe gastric ulceration typi-agus was decreased by 132 adrenoceptor stimula- cally accompanies GERD in equine and treatmenttion. Clenbuterol, which is used commonly in of the primary problem (gastric emptying dis-horses, caused relaxation of rat esophageal order) and the secondary GERD includes sup-smooth muscle (de Boer et al 1993), but it is pression of gastric acid secretion and promotionnot known whether drugs such as clenbuterol of gastric emptying. The drugs used to promoteor terbutaline will similarly affect the equine gastric emptying also increase LES tone, furtheresophagus. limiting gastroesophageal reflux. Drugs used to suppress gastric acidity are covered in the sec- tion on the stomach. Drugs used to promote LESOxytocin tone include bethanecol, metoclopramide andIt has been suggested that oxytocin would reduce motilides.esophageal contractility and be useful in thetreatment of esophageal obstruction in horses(Hance et al 1997). In one study in nine horses, Bethanecholintraluminal esophageal pressure was reduced Bethanechol has been used in humans to improveduring i.v, infusion of oxytocin at dose rates of LES tone and reduce gastroesophageal reflux0.11, 0.22 and 0.44IU/kg (Meyer et al 2000). (Sondheimer & Arnold 1986) but has now beenIncreasing the dose increased the duration of the replaced by other treatments that produce fewerresponse. Signs of abdominal discomfort were side-effects. Bethanechol has been used, in con-observed in two horses at the lowest dose. Neither junction with acid-suppressing drugs, by thethe effect of oxytocin on striated or smooth mus- author to treat GERD in foals and adult horses.cle nor its mechanism of action is known. Its Treatment was associated with healing of theclinical efficacy is presumed but has not been esophagitis but it is not known whether thedocumented. responses could be attributed to the effects of bethanechol or reduced acidity. Its effects on theAcepromazine LES in horses have not been examined but bethanechol does accelerate gastric emptying inAcepromazine maleate is advocated for use in horses (Ringger et al 1996), which may explainthe treatment of esophageal obstruction but, aside the positive clinical response to treatment withfrom its sedative effects, there is no direct evi- bethanechol in foals and horses with GERD.dence that it promotes esophageal relaxation. Inone report using barium contrast radiography,acepromazine appeared to cause mild segmental Metoclopramidedilatations in the thoracic esophagus (King et al1990). The duration of effect of acepromazine is Metoclopramide increases LES sphincter tone2 to 3h and it may be useful in the treatment of but it is of questionable efficacy in the treatmentobstructions that do not resolve quickly. The dose of GERD in humans (Bellissant et al1997, Granderate is 0.02-D.04 mg/kg i.v, or i.m, et aI1992). Doses as low as 0.1 mg/kg p.o. three times a day have been suggested but no data sup- port the use of metoclopramide in the treatmentDRUGS USED TO TREAT REFLUX of GERD in horses or foals. The author has usedESOPHAGITIS metoclopramide, at a dose rate of 0.3mg/kg p.o.Gastroesophageal reflux disease (GERD) is a fre- four times a day, with apparent success in onequent disorder in humans but is uncommon in foal with GERD but, as has been reported in the
  • 98. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 97human medical literature, adverse neurological humans, lie deep within the wall of the stomach.effects accompanied treatment. When the dose Smooth muscle in the wall of the stomach becomeswas reduced to 0.2mg/kg, GERD and delayed progressively thicker towards the antrum andgastric emptying recurred. pylorus. Contractions progress from the body to the pylorus, and strong, circumferential propulsive contractions in the antrum result in a golf-Motilides ball-sized bolus of ingesta moving into the duo- denum. Afferent fibers arising from the vagusThere are few reports of the effects of motilides nerve and from splanchnic and mesentericon LES tone. Data from these reports, on small nerves can detect noxious substances (such asnumbers of individuals, suggest that erythromy- hydrochloric acid) and stimulate mucosal arterio-cin does increase LES pressure and may prevent lar vasodilation (Holzer 1998). These extrinsicgastroesophageal reflux. afferent neurons can be stimulated by capsaicin and the effector substance for vasodilation is NO. This system may be an appropriate target forSTOMACH pharmacological intervention in the future. Hydrochloric acid is secreted by parietal cellsThe principal functions of the stomach are to via H+,K+-ATPase pumps (proton pumps), ofserve as a reservoir for ingesta, to deliver small which there are more than one million per cell.boluses of ingesta to the small intestine, to begin The H+ ,K+ -ATPase pumps utilize the phospho-the digestion of proteins and to serve as a first rylation of ATP to exchange water-solvated pro-line of defense against enteric pathogens. It is tons (protonated water, hydroxonium ion, H 30+)these latter two functions for which the secretion for potassium ions. In conjunction with parallelof hydrochloric acid and pepsin are physiologi- potassium and chloride ion conductances, thiscally most relevant. The stomach also has impor- ATPase is responsible for the secretion of hydro-tant endocrine activities, such as the release of the chloric acid into the secretory canaliculus of thehormone gastrin, and influences the physiology parietal cell, the enclosed space reaching a pH ofof other segments of the intestinal tract via both near 1.0 (Rabon & Reuben 1990). In the restingendocrine and neural pathways. The functions of parietal cell, these pumps reside within the mem-the stomach that are most often manipulated branes of vesicles in the cell cytoplasm. Whenusing pharmacological agents are motility and activated by histamine and gastrin, the parietalemptying, secretion of hydrochloric acid and the cells alter their shape and the vesicles mergebarrier to hydrochloric acid and pepsin on the with the outer cell membrane to form secretorysurface of the glandular mucosa. canaliculi. The equine stomach is lined dorsally by a strat- The stomach secretes hydrochloric acid underified squamous epithelium and ventrally by a the influences of vagus nerve stimulation, gastringlandular epithelium, which have different func- and histamine. Histamine is the most potenttions and susceptibility to peptic injury. The squa- stimulus of gastric acid secretion in some animalmous portion of the stomach has no secretory or species studied, such as the horse (Kitchen et alabsorptive function and appears to serve as a reser- 1998a). Histamine is released by mast cells andvoir for ingesta. The gastric glandular mucosa has enterochromaffin-like cells that are immediatelyan array of secretory and endocrine functions. adjacent to the parietal cells. Histamine interacts Gastric motility is initiated via impulses from with two distinct subsets of histamine H 2 recep-the vagus nerve. In horses, the vagus nerve travels tors on the parietal cell membrane, initiating aalong the esophagus, through the diaphragm and, series of reactions that result in the phosphoryla-once through the diaphragm, immediately enters tion of protein kinases and increased intracellularthe wall of the stomach, whereupon it divides calcium within the parietal cell. This, in tum,into several branches that in the horse, unlike in results in transformation and translocation of the
  • 99. 98 EQUINE CLINICAL PHARMACOLOGYcoiled tubular vesicles in which the H+,K+- accompanied by gastroesophageal reflux andATPase pumps reside to the secretory canaliculi. esophagitis. The gastric squamous mucosa isGastrin is released into the blood by G cells in the spared from exposure to hydrochloric acid andstomach and acts as a true hormone. Gastrin pepsin by the frequent consumption of roughagereaches parietal cells via the blood and initiates a by adult horses (Murray & Schusser 1993), orseries of reactions that result in increased intra- milk by foals (Sanchez et a11998), which absorbscellular calcium. Gastrin also interacts with gastric gastric secretions and is probably accompaniedmast cells and enterochromaffin cells, promoting by salivary bicarbonate, which can neutralizethe release of histamine. In addition to stimulat- hydrochloric acid. The continuous secretion ofing hydrochloric acid secretion by the stomach, hydrochloric acid by the equine stomach makesgastrin appears to stimulate the secretion of horses particularly susceptible to damage to thewater, sodium, chloride and bicarbonate from the gastric squamous mucosa. Periods of prolongedpancreas into the duodenum, some of which nor- high gastric acidity (pH < 2.0) were created inmally refluxes into the stomach. The primary horses using a protocol of alternating 24h peri-physiological inhibitor of gastric acid secretion is ods of feed deprivation with free choice Timothysomatostatin; prostaglandins E] and E2 and epi- hay, which consistently resulted in erosion anddermal growth factor appear to modulate acid ulceration, often severe, of the gastric squamoussecretion to a lesser degree. epithelial mucosa (Murray & Eichorn 1996). The equine stomach secretes hydrochloric acid Erosions, sometimes bleeding, were seen aftercontinuously, even when the horse is not eating 48h of cumulative feed deprivation and ulcers(Campbell-Thompson & Merritt 1990). Gastric acid were seen consistently after 96 h. The gastricsecretion is pronounced even in neonatal foals glandular mucosa is less susceptible to direct pep-(Sanchez et aI1998). Gastric acidity is lowest when tic injury than the squamous mucosa, because itthe horse eats, because eating stimulates the secre- has evolved elaborate mechanisms (mucus/bicar-tion of bicarbonate-rich saliva, which can neutral- bonate barrier, prostaglandins, mucosal bloodize some gastric acid, and roughage absorbs gastric flow, cellular restitution) to protect itself fromsecretions so they do not contact the mucosal sur- peptic injury (Hojgaard et al 1996). The mucus-face. Once a horse stops eating, the gastric acidity bicarbonate barrier consists of a thin (200IJ.m)can rapidly increase, with the pH falling to below mucus layer that has hydrophobic characteristics2.0, and the acidity will remain high for as long as and into which bicarbonate ion is secreted.the horse does not eat (Murray & Schusser 1993). Surface-active phospholipids in the mucus repel Peptic injury refers to damage to the mucosa aqueous hydrochloric acid and the bicarbonateof the alimentary tract by hydrochloric acid that is trapped within the mucus layer creates aand pepsin. Bile acids can contribute to peptic pH gradient from the lumen of the stomach to theinjury but both bile acids and pepsin require an surface epithelial cells in the order of magnitudeacidic milieu to cause injury. The stomach is the of 100000. These attributes provide a substantialpredominant site of peptic injury in foals and barrier to the back-diffusion of hydrochloric acid.horses, although gastric secretions may also injure Mucosal blood flow is probably the most impor-the esophagus and duodenum. Protection of the tant element of gastric mucosal protection. NOequine esophageal and gastric squamous mucosae and prostaglandins are key regulators of mucosalfrom peptic injury is dependent upon limited blood flow, with NO probably being the primaryexposure to acidic gastric secretions, because there factor.is no surface barrier to hydrochloric acid and In studies in which periods of feed deprivationthese epithelia have limited properties to prevent induced ulcers in the squamous mucosa, no lesionspeptic injury. The esophagus is normally spared were induced in the glandular mucosa. However,exposure to gastric secretions by the LES,although the prevalence of lesions in the glandular mucosadisorders that result in impaired gastric empty- of the antrum was 57% in one report of 162 horsesing (pyloric ulcers, duodenitis) are frequently (Murray et al 2001). In humans, most lesions in
  • 100. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 99the gastric mucosa are caused by Helicobacter pylori, removal of the acid allows healing to proceedNSAIDs and alcohol. In most cases, the cause of unimpeded.lesions in the equine gastric glandular mucosa isundetermined. H. pylori or similar bacteria havenot been identified in equine gastric mucosa. AntacidsNSAIDs can cause injury to the gastric glandular Antacids reduce gastric acidity by neutralizingmucosa at high doses in foals and horses. existing acid. In an environment with an acidic The physiological impact of hydrochloric acid pH, these compounds exchange cations for hydro-on the lining of the stomach, as well as the esoph- gen ions, thus removing hydrogen ion activityagus and duodenum, is based on the hydrogen from the gastric milieu. Most antacids are basedion activity and not how much acid is present. on a combination of aluminum and magnesiumGastric acidity is measured by pH, which is a log- hydroxides or calcium carbonate. The aluminumarithmic measurement of hydrogen ion activity hydroxide/magnesium hydroxide antacids neu-(pH = -loglO[aH+ D. Hydrogen ion activity is deter- tralize acid by combining with a water-solvatedmined by both the hydrogen ion concentration proton (H30+) to yield water plus a metal ion:and the ionic strength of a solution. The greaterthe ionic strength, the lower the pH of a solution Metal (OHh + 2H 30+ ~ Metal/" + 4H 20with a given concentration of acid. Another per- Calcium carbonate interacts with hydrooxoniumspective on pH is obtained applying Stewarts ions to form carbonic acid and a calcium ion, withstrong ion theory, in which the balance of strong carbonic acid yielding CO 2 and water:ions (sodium, hydroxonium, chloride and hydrox-ide) determines the acidic activity of a solution. CaC03 + 2H30+ ~ Ca2+ + 2H20 + H 2C03 ~H20 + CO 2Consequently, when evaluating a report on adrugs effect on gastric acidity, the effect of the In these reactions within the acidic gastric milieu,drug on pH is probably more relevant, as far as the remaining cations (aluminum, magnesium,the gastric mucosa is concerned, than data on calcium) combine principally with chloride ions.acid output, which are attractive from physiolog- Antacids can effectively reduce gastric acidityical and quantitative perspectives. but only briefly. The dose and gastric emptying limit the magnitude and duration of the effect of antacids on gastric acidity. In one study examin- ing administration of 180 ml of a combination ofDRUGS AFFECTING GASTRIC aluminum and magnesium hydroxide suspen-ACIDITY sion, gastric pH was increased for 45 min at mostWhen considering using acid-suppressive ther- (Murray & Grodinsky 1992). In another study,apy in foals or adult horses, one must bear in 240 ml of a aluminum plus magnesium hydroxidemind that the horse is a continuous secretor of suspension increased gastric pH for 2 h (Clark et alhydrochloric acid (Campbell-Thompson & Merritt 1996). Therefore, liquid antacid products must be1990), there is no diurnal pattern to gastric acidity given both in large volumes (240ml) and very fre-(Murray & Schusser 1993) and that the mecha- quently (6-12 times daily!) to be effective in pro-nism of action of an acid-suppressive agent and moting ulcer healing. Some clinicians believe thatthe dose administered will affect the potency and clinical signs are relieved at lower doses givenduration of acid suppression. Additionally, doses less frequently, but these empirical claims haveof an acid-suppressive agent that appear to result not been confirmed. Feed additives that containin improved clinical signs may not result in ulcer antacids are popularly considered to be helpfulhealing. Importantly, suppression of gastric acid- in controlling gastric ulcers in horses but thereity does not stimulate ulcer healing but is permis- are no supportive data. Also, an acid-neutralizingsive to ulcer healing. Mechanisms of healing are effect is most desirable when the stomach isinitiated with the onset of mucosal injury and empty, not when it is full, because gastric pH is
  • 101. 100 EQUINE CLINICAL PHARMACOLOGYnaturally high when horses ingest feed (Murray degree (to 5.3 :±: 1.1) than the lower dose rate& Schusser 1993). (4.1 :±: 1.2) during the 6h following administra- In humans, antacids are used primarily to con- tion (Murray & Grodinsky 1992). The effects oftrol symptoms of dyspepsia (heartburn, upset famotidine (0.5, 1.0 and 2.0mg/kg) were alsostomach) and these agents are not considered pri- investigated in the same study; gastric fluid pHmary therapy for the treatment of ulcer disease. was >6 for longer during the 6 h after administra-Only subjective assessments are available for tion of ranitidine than for all doses of famotidine.evaluating the effects of antacids on signs of gastric In a study in which an indwelling pH electrodediscomfort in horses and the author has noted was used to measure gastric pH in horses feda substantial disparity between the effectiveness grass hay, oral ranitidine 6.6 mg/kg three timesof antacids reported by owners and trainers and a day resulted in a median 24h pH of 4.8, com-endoscopic findings. The latter observations at pared with a median pH of 3.2 in horses fed hayleast indicate ineffectiveness of antacids in treat- without treatment (Murray & Schusser 1993).ing peptic lesions in horses. Sanchez et al (1998) investigated the effects of i.v. (2.0mg/kg) and p.o. (6.6mg/kg) ranitidine on gastric acidity in young foals. Mean intragas- tric pH significantly increased for 5h after i.v,Histamine H2 receptor antagonists administration compared with baseline data.The histamine H 2 receptor antagonists (H 2 anta- After p.o. administration, gastric pH significantlygonists) inhibit hydrochloric acid secretion by increased for up to 8 h.competing with histamine for receptor sites on Cimetidine is used widely in equine practicethe parietal cell (Katz 1991). Histamine is the but its effect on gastric acidity has received onlymost potent stimulus for hydrochloric acid secre- limited investigation. In one study, cirnetidine wastion and, because occupation of the receptor site administered p.o. to five horses at 8.8 mg/kg andis by competitive inhibition, the greater the was reported to have increased gastric pH to >3.6concentration of H 2 antagonist at the receptor for 8 h after administration (Sangiah et al 1988).site, the greater and more prolonged the degree Unlike other studies in which gastric fluid samplesof suppression of hydrochloric acid secretion. were collected at 15min intervals or gastric pH wasThe H 2 antagonists approved for p.o. use in measured continuously by pH electrode, gastrichumans in the USA are cimetidine, ranitidine, samples were only collected every 2 h. Becausefamotidine and nizatidine. All are available as gastric pH can fluctuate considerably and becausegeneric products and are also available over-the- high pH recordings can occur spontaneouslycounter in the USA. The over-the-counter products (Kitchen et al 1998b), the results of this studyare sold in lower strengths, usually one-quarter should, therefore, be interpreted with caution.strength, than the prescription forms. Injectable An important consideration when evaluatingformulations of cimetidine, ranitidine and famo- the effect of an acid-suppressive drug is that intra-tidine are also available. gastric pH in an individual animal will tend Several studies have examined the effects of toward the low (1.0-3.0)or high (5.0-7.0)pH range.histamine H 2 antagonists on gastric acid secretion These are the ranges of greatest pH stability inand gastric acidity in horses. In the first such study the stomach. At low pH, the hydroxonium ion isreported, ranitidine administered i.v, to young the predominant cation, whereas at higher pH thehorses at 0.5 mg/kg inhibited gastric acid output sodium ion predominates. In fact, in one report,but had no significant effect on gastric acidity pH values paralleled the sodium concentration of(pH) (Campbell-Thompson & Merritt 1987). gastric fluid (Merritt et al 1996). In the pH rangeComparison of two dosages of ranitidine (4.4 3-5, however, there is a transition between theand 6.6 mg/kg), administered by nasogastric predominant cation and pH tends rapidly eithertube, on gastric fluid pH revealed that the higher to decrease to <3.0 or to increase to >5.0. Thisdose rate increased gastric fluid pH to a greater has been noted in horses that had not been
  • 102. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 101Figure 6.1 Frequency histogram of pH recordings from a horse administered oral ranitidine at 4.4 mg/kg. The horse hadTimothy hay available ad libitum. Recordings of pH were made using an indwelling glass pH electrode in the stomach. Notice thatthe majority of pH recordings are <3.0 or >5.0. Recordings >5.0 reflect the effect of the drug on suppressing gastric acidsecretion and pH values rapidly decreased to <3.0 when the effect of the drug subsided.treated with acid-suppressive drugs and, in the minimally and for a short period (Fig. 6.2). Thecontext of measuring gastric pH, as a response to author has noted that the type of response appearsadministration of an Hz antagonist (Fig. 6.1). In to be both horse and dose dependent. For example,the former situation, the increase in pH probably one horse had an attenuated and short-livedresults from reflux of sodium-rich duodenal con- increase in gastric fluid pH after p.o. administra-tents into the stomach, whereas in the latter the tion of 6.6mg/kg ranitidine or 2.0mg/kg farno-increased pH results from suppression of secre- tidine, whereas another had pronounced andtion of hydrochloric acid. When an Hz antagonist, prolonged increases in gastric fluid pH (Fig.6.3).such as ranitidine, is administered into the This variability in response to the Hz antago-stomach, the pH of the gastric contents typically nists probably reflects the variable and relativelyincreases to >5.0 within 45-60 min. When the poor oral bioavailability of these drugs in horses.effect of the drug wanes, as a result of decreased In one study of five horses, the mean oral bioavail-blood concentrations, pH does not gradually ability of cimetidine, ranitidine and famotidinedecrease but falls rather precipitously to <3.0. were 30%, 13% and 24%, respectively (Duran & Horses have individually characteristic gastric Ravis 1993). In another report, the mean oral bio-pH responses to oral ranitidine and famotidine, availability of cimetidine was 14% (range 7-22%)with three patterns of response observed (Murray (Sams et al 1997). When administered p.o. at& Grodinsky 1992). A complete response is one 3.3 mg/kg, cimetidine could not be detected inwhere gastric fluid pH increases to 7.0 or greater plasma (Smyth et a11990). Pharmacokinetic stud-and remains >6.0 for 4-10 h. An intermediate ies of ranitidine in six foals and six adult horsesresponse is one where there is a biphasic increase in revealed a mean oral bioavailability of 38%in foalspH (increase, decrease, then increase) and a poor and 27% in adult horses. The half-life of cimetidineresponse is one where gastric fluid pH increases ranges from 1 to 2.2h (Sams et al1997, Smyth et al
  • 103. 102 EQUINE CLINICAL PHARMACOLOGYFigure 6.2 Gastric fluid pH measurements after famotidine was administered by nasogastric tube at dose rates of 0.5 (.), 1.0(_) and 2.0 mg/kg (A) to a horse that had not been fed for 18 h. Gastric fluid was aspirated through a weighted nasogastric tubeat 15 min intervals for 1 h before and from 45 min after administration. The gastric fluid pH dose-response in this horse typify thepoor (0.5 mg/kg), intermediate (1.0 mg/kg) and good (2.0 mg/kg) responses to orally administered H2 antagonists in horses.1990). Cimetidine is excreted in the urine of horses treatment. In vehicle-controlled studies in whichas both the parent drug and the sulfoxide (Sams ulcers were induced experimentally by transendo-et al1997). scopic cautery (MacAllister et al1994) or flunixin Administration of Hz antagonists i.v, is recom- (MacAllister & Sangiah 1993),cimetidine (18 mg/mended for foals and horses with conditions in kg p.o. every 12 hours) or ranitidine (4.4mg/kgwhich gastric emptying or intestinal absorption p.o. every 8 hours), respectively, failed to enhanceis impaired. After i.v, administration of 2.0 mg/kg healing. In another study, ranitidine 6.6mg/kgranitidine to foals, mean intragastric pH increased three times a day prevented the induction ofsignificantly to >4.0 for 5 h. The effect of i.v, cime- ulcers in a feed-deprivation model (Murray &tidine on gastric pH in horses has not been Eichorn 1996).reported, although pharmacokinetic data would No large-scale, controlled clinical trial to eval-support its use (Sams et al 1997, Smyth et al uate the efficacy of Hz antagonists on ulcer healing1990). The authors clinical experience using in horses have been carried out; yet clinical expe-cimetidine i.v, at a dose rate of 6.6mg/kg three to rience supports the use of these drugs in practice.four times a day has been consistently positive, What is clear from the experimental data is that,with ulcer healing progressing at a rate consistent to be effective, Hz antagonists must be adminis-with excellent suppression of gastric acidity. tered at doses that can be expected to increase There is conflicting evidence regarding the gastric pH for 4-8 h. Ranitidine has been studiedeffect of Hz antagonists on the healing of gastric most extensively and an oral dosage of 6.6 mg/kgulcers in foals and horses. In clinical reports where three times a day is supported by several studiesthere were no control horses, p.o. administration (Murray & Eichorn 1996, Murray & Grodinskyof ranitidine at 6.6 mg/kg for 3 weeks was associ- 1992,Murray & Schusser 1993,Sanchez et al1998).ated with complete ulcer healing in up to 90% Even a 33% reduction in dose, to 4.4 mg/kg, hasof horses (Furr & Murray 1989).Cessation of clin- consistently appeared to have an unpredictableical signs attributed to the gastric ulcers was effect on gastric pH increases and was often inef-reported to occur within a few days of beginning fective altogether (Murray & Grodinsky 1992).
  • 104. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 103Figure 6.3 Gastric fluid pH in response to the oral administration of famotidine or ranitidine to two horses. (a) Administration of2 mg/kg famotidine stimulated no response in horse 1 whereas horse 2 has a substantial increase in gastric fluid pH that persistedfor more than 10 h. (b) Administration of 6.6 mg/kg ranitidine had an intermediate effect in horse 1, gastric fluid pH intermittentlyincreased and decreased. By comparison, horse 2 showed a substantial increase in gastric fluid pH that persisted for more than10h. The pH response in horse 1 may also reflect in part, reflux of duodenal contents into the stomach in addition to. or insteadof, a response to ranitidine.Also, concurrent consumption of roughage (MacAllister et al 1994). In one published report,appears to enhance the effect of ranitidine on cimetidine 20 mg/kg three times a day was inef-increasing gastric pH (Murray & Schusser 1993). fective in healing or preventing gastric ulcers in Cimetidine is used widely in equine veteri- thoroughbred racehorses in training (Nieto et alnary practice but there are sparse data to support 2001). The author has administered cimetidine, atits use. Cimetidine at 18mg/kg p.o. three times doses ranging from 20 to 25 mg/kg p.o. threea day was no more effective than control in times a day with variable success. These horseshealing electrocautery-induced ulcers in horses were kept out of work during treatment.
  • 105. 104 . EQUINE CLINICAL PHARMACOLOGY (Cederberg et al 1989), 80% (Gerloff et al 1996)H+,K+-ATPase inhibitors and 75% (Huber et al 1996), respectively.There are several H+,K+-ATPase systems in mam- The greater bioavailability of lansoprazole andmalian and plant cells and the gastric form of pantoprazole may relate to greater first-passH+ ,K+ -ATPase can be found in a variety of tissues metabolism of omeprazole.(Scarff et al 1999). However, the H+,K+-ATPase A paste formulation of omeprazole is approvedinhibitors, also referred to as proton pump for use in horses. The bioavailability of the omepra-inhibitors, interact specifically with parietal cell zole is lower from the paste formulation thanH+ ,K+ -ATPase because of their chemical struc- from the enteric-coated granules. More omepra-tures and the uniquely highly acidic environment zole in the paste was absorbed when adminis-of the secretory domain of the parietal cell H+, K+- tered to horses that had feed withheld than whenATPase. The H+,K+-ATPase inhibitors available given to horses that were fed (Daurio et al 1999).for treatment of peptic disorders (omeprazole, In humans, approximately 80% of omeprazolelansoprazole, pantoprazole, rabeprazole) are and pantoprazole are excreted in urine, with thesubstituted benzimidazoles that are transferred remainder excreted in feces. Lansoprazole israpidly from the blood into the acidic secretory primarily excreted in feces (up to 70%), indicat-canaliculi of the parietal cells (Besancon et aI1997). ing substantial hepatic metabolism and biliarySubstituted benzimidazoles are weak bases, so excretion.they become charged within the acidic canaliculi Omeprazole has been studied extensivelyand vesicles of the parietal cells, consequently in horses. Enteric-coated omeprazole granules,trapping and accumulating the drug within administered to mares at 0.7 mg/kg once daily viathe parietal cell. These drugs then undergo a series nasogastric tube, resulted in significant increasesof chemical transformations that require a in basal and pentagastrin-stimulated gastric pHhighly acidic solution, resulting in a cationic after the fifth dose (Andrews et aI1992). However,sulfenamide; this forms covalent disulfide bonds there was no effect on acid output or gastric pHwith sulfhydryl groups of cysteine molecules, within 8 h after the first dose. When enteric-which are part of the catalytic subunit of coated omeprazole granules were administeredthe H+ ,K+ -ATPase and which face the acidic at 1.4 mg/kg once daily, the effects on basal andsecretory canaliculi. The H+ ,K+ -ATPase inhibitors pentagastrin-stimulated acid output and gastricbecome bound irreversibly to the catalytic portion pH were more pronounced and prolongedof the H+ ,K+ -ATPase enzyme system and block (Jenkins et aI1992a). On the fifth day of adminis-the activity of the enzyme until new enzyme tration, gastric pH was consistently >6.0 and gas-is generated. The magnitude of inhibition of tric pH remained increased 24h after the sixthacid secretion is dose dependent, so that at daily dose. As with the 0.7mg/kg dose, gastric pHhigher doses more catalytic sites are blocked; was unaffected within 8 h after the first dose ofbecause of this mechanism of action, these drugs omeprazoIe. Whereas omeprazole administered ascan inhibit acid secretion by up to 99% for 24h enteric granules did not have an immediate effector longer. to significantly increase gastric pH (Jenkins et al Omeprazole, lansoprazole and pantoprazole 1992a), within 1h of administering the secondare highly unstable within the acidic environment dose (4mg/kg) of the approved omeprazole pasteof the stomach and lose their activity prior to formulation, gastric pH increased from basal lev-absorption in the small intestine. The formulations els to >6.0 (Merritt et al 2002).of omeprazole and lansoprazole developed for Omeprazole administered i.v. (0.5mg/kg onceuse in humans utilize an enteric coating to pro- daily) had a pronounced effect on basal acid out-tect the drug as it passes through the stomach. put and gastric pH within 2 h of administration,Pantoprazole is available as a delayed-release and basal and pentagastrin-stimulated acid out-tablet. The bioavailability of omeprazole, lanso- put were significantly inhibited 24h after theprazole and pantoprazole in humans is 35-60% fourth consecutive dose (Jenkins et al 1992b).
  • 106. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 106Omeprazole administered i.m. (0.25 and 1.0mg/ measured 24h after the fifteenth dose. Gastric pHkg) similarly inhibited basal and pentagastrin- was significantly increased, usually to >6.0.stimulated acid output and increased gastric pH, The same paste formulation of omeprazole haswith a greater response to the higher dose rate been examined in several pre- and postapproval(Sandin et aI1999). Peak plasma levels of omepra- studies. A dose-confirmation trial was conductedzole were measured 20 min after administration in horses in simulated, yet intense, race trainingand thereafter plasma levels rapidly declined (half- that both induced and maintained gastric ulcerslife of 45 to 60 min). Inhibition of acid secretion per- (Andrews et al 1999). The study had two 4 weeksisted for many hours after the plasma levels had phases: active treatment and prevention of recur-declined to barely detectable. The bioavailability rence. During this time, the horses were kept inof the i.m, administered omeprazole was 75%. intensive training. After 4 weeks of treatment Clearly, the formulation and route of adminis- with omeprazole paste at 4 mg/kg daily, ulcerstration of omeprazole in horses strongly influence were improved (defined as an ulcer score lowerthe onset, magnitude and duration of acid sup- than pretreatment) in 92% of the horses receivingpression. In one report, the inhibitory effects of omeprazole paste compared with only 32% of thedifferent formulations of omeprazole on basal sham-treated horses. Ulcer healing, defined as anand pentagastrin-stimulated acid secretion were ulcer score of zero, occurred in 77% of treatedexamined (Haven et aI1999). These formulations horses compared with 4% of sham-treated horses.included an injectable formulation administered In the prevention phase of the study, ulcersi.v. (O.5mg/kg), acid-stable omeprazole granules did not recur in 84% and 88% of healed horsesgiven by nasogastric tube (1.5 and 5.0mg/kg), subsequently treated daily with omeprazoleacid-stable omeprazole granules given p.o. as a paste at 2 or 4mg/kg, respectively, whereaspaste (1.5mg/kg) and omeprazole powder given ulcers persisted, recurred or became more severep.o. as a paste (1.5 and 3.0mg/kg). The i.v, formu- in 92% of sham-treated horses. Field clinical trialslation performed as expected, markedly inhibit- have further documented the efficacy of thising acid secretion and increasing gastric fluid pH. omeprazole oral paste formulation, at a dose rateThe acid-stable omeprazole granules given by of 4mg/kg once daily, for treatment of gastricnasogastric tube at 5.0mg/kg once daily also ulcers in foals and adult horses. Other studiesmarkedly suppressed acid secretion and increased have shown that this paste formulation isgastric fluid pH; however, this formulation at accepted readily by foals and horses and appears1.5mg/kg had only a moderate effect and less not to produce adverse effects, including infertil-of an effect than reported previously for 1.4mg/ ity in stallions.kg (Jenkins et al 1992b). The paste formulationsof omeprazole all decreased acid output andincreased gastric fluid pH, although to a lesser Other drugs that affect gastricdegree than the i.v.administered omeprazole and aciditythe granule formulation given at 5.0 mg/kg bynasogastric tube. Misoprostol, an analog of prostaglandin El , has In another report, the paste formulation of acid-suppressive effects in horses when adminis-omeprazole approved for equine use markedly tered p.o. at 5 f.Lg/kg (Sangiah et al 1989). How-and equivalently suppressed gastric acid secretion ever, it is thought that the primary effect ofand increased gastric fluid pH at p.o. dosages of misoprostol is based on the enhancement of4.0 and 5.0mg/kg once daily (Daurio et aI1999). mucosal protection from peptic injury. Misopros-Data were collected after the fifth, tenth and fif- tol can cause abdominal cramping and diarrheateenth consecutive daily dosages; acid output (H+ in humans and for this reason it may be unsuit-as f.Lmol/15 min per kg) was 99% suppressed able as a treatment for gastric ulcers in equines.when measured 8 h after the fifth dose, 95% when In addition, misoprostol offered no advantagemeasured 16h after the tenth dose and 90% when over omeprazole in preventing or healing
  • 107. 108 EQUINE CLINICAL PHARMACOLOGYNSAID- induced gastric or duodenal ulceration in Favorable clinical results have been obtainedhumans (Hawkey et aI1998). with p.o. doses of 1D-20mg/kg three times a day. The somatostatin analog octreotide can decrease Sucralfate can be administered concurrently withgastric acid secretion. In one report, octreotide, an Hz antagonist. Concurrent administration maygiven s.c. to horses at dose rates of 0.1, 0.5, 1.0and reduce the absorption of the Hz antagonist by5.0 f.Lg/kg, increased gastric pH to >5.0 compared 10% but this has not appeared to affect efficacywith baseline values (consistently <2.7) (Rabon in humans (Mullersman et aI1986). Importantly,& Reuben 1990). The duration of effect was dose sucralfate can substantially interfere with thedependent and ranged from 2.4 to 5.4 h. absorption of other drugs, particularly fluoro- quinolones, and thus its use concurrently with other medications should be determined on aDRUGS ACTING ON GASTRIC case-by-case basis.MUCOSAL PROTECTIONSome agents do not suppress gastric acidity (with Misoprostolthe exception of aluminum-containing antacids,which have some mucosal protection properties) The prostaglandin E1 analog misoprostol enhancesbut enhance the intrinsic protection of the gastric mucosal protection by promoting mucosal bloodmucosa against hydrochloric acids and pepsin. flow and possibly increasing gastric mucus (DajaniImportantly, for the equine stomach, the squa- & Agrawal 1989). Misoprostol has demonstratedmous mucosal lining lacks the surface barriers to clinical effectiveness in preventing duodenalpeptic injury that are found within the glandular ulcers in humans, particularly NSAID-inducedmucosa; therefore, agents that affect mucosal ulcers (Roth 1990). In one study in horses anotherprotection will have little to no impact on the prostaglandin E1 analog limited phenylbutazone-glandular mucosa. induced gastrointestinal mucosal damage (Collins & Tyler 1985).Sucralfate Aluminum-containing antacidsSucralfate, the major components of which aresucrose octasulfate and aluminum hydroxide, is In addition to their acid-neutralizing properties,effective in the treatment of peptic ulcers in antacids containing aluminum appear to havehumans (McCarthy 1991), although the healing properties that can protect the gastric mucosa fromof duodenal ulcers took much longer than with peptic injury. These antacids have been shown toHz antagonists. Clinical experience suggests that protect the mucosa against NSAID- or ethanol-sucralfate can promote the healing of lesions in induced injury by a mechanism that involves thethe gastric glandular mucosa of horses. The mech- production of NO, which promotes mucosal bloodanism of action probably involves adherence flow (Konturek et aI1992).to the ulcerated mucosa, stimulation of mucussecretion and enhanced mucosal blood flow and NO agonistsprostaglandin E synthesis. These are all factorsrelevant to the glandular mucosa and it is doubt- Gastric mucosal blood flow is important in main-ful that sucralfate is effective in treating ulcers taining mucosal acid-base balance and in main-in the equine gastric squamous mucosa. In fact, taining mucosal barriers to hydrochloric acid.lesions in the squamous mucosa can develop NO is the primary promoter of mucosal perfu-while a horse is undergoing treatment with sion. Inhibitors of NO synthase have been shownsucralfate. to augment gastric mucosal injury and to impair No studies have been performed in horses to healing of gastric ulcers. Conversely, administra-determine an appropriate dose of sucralfate. tion of L-arginine, a substrate of NO synthase, has
  • 108. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 107been shown to enhance gastric mucosal protec- with suspected gastric-emptying disorders. Meto-tion and healing in laboratory animals. Positive clopramide has the potential to cause severe exci-effects of L-arginine have been demonstrated tation in foals and horses because of its ability towhen it was administered parenterally or p.o. cross the blood-brain barrier and its inhibitory(Brzozowski et al 1997). Aluminum-containing effects on dopamine receptors. Therefore, theantacids and sucralfate are thought to provide challenge is to administer a dose that effectivelygastric mucosal protection by promoting NO stimulates propulsive motility whilst avoidingproduction, resulting in mucosal vasodilation adverse effects. Foals have been given dosesand increased blood flow. ranging from 0.1 to 0.25mg/kg three to four times a day. However, there are few data to support the use of metoclopramide to promote gastric empty-DRUGS ACTING ON GASTRIC ing in horses. In one report, metoclopramide,MOTILITY given to horses, as a slow infusion at a dose rateBethanechol of 0.125mg/kg, was shown to increase gastric emptying in a low-dose endotoxin model (DohertyThe cholinergic agonist bethanechol has been used et al 1999). Bethanechol appears to be preferable to stimulate gastric emptying in foals and horses to metoclopramide because it is effective and haswith duodenitis, pyloric stenosis and pyloric limited side-effects.ulceration. Bethanechol also has been used tofacilitate gastroduodenoscopy in foals and horses. CisaprideIn a study in which scintigraphy was used tomeasure emptying rates of 99Tc-labeled sulfur Cisapride is highly effective in increasing gastriccolloid incorporated into egg albumin adminis- emptying in humans. In one report, cisapridetered into the stomach of horses, i.v, administra- given p.o. at O.4mg/kg had no effect on gastriction of 0.025mg/kg bethanechol increased the emptying in normal horses but improved gastricrate of gastric emptying significantly (Ringger et al emptying in horses given low-dose endotoxin1996). In saline-treated horses, the average time (Valk et al 1998). However, other investigatorsfor emptying of half of the solid gastric contents found that the same dose of cisapride was(Tso) was 90min, compared with a Tso of 30 min absorbed highly erratically and generally poorlyin bethanechol-treated horses. Excessive saliva- (Steel et al1998).tion was noted in some of the horses adminis-tered bethanechol i.v. Clinically, bethanechol isgiven s.c, at a dose rate of 0.02mg/kg or p.o. at Motilides0.35mg/kg three times a day. Cholinergic side- Similar to other prokinetic drugs, erythromycineffects are not noted at these doses. Bethanechol has been shown to improve gastric emptying inhas been given to some horses chronically (weeks humans and laboratory animals. In horses, eryth-to months) without apparent adverse effects. It is romycin lactobionate, at both 0.1 and 1.0mg/kg,not known whether the drug remains effective reduced the Tso of solid-phase gastric emptyingwhen given chronically or whether horses might (Ringger et al1996).become refractory. Indications for chronic admin- Erythromycin should probably not be the firstistration of bethanechol include pyloric stenosis choice to treat delayed gastric emptying in horsesand chronic ulceration of the pylorus. because of its potential adverse effects (clostridial colitis, hyperthermia) even at low doses.BenzamidesMetoclopramide Xylazine and detomidineMetoclopramide has been used effectively in Xylazine and detomidine are (l2 adrenoceptorhumans to improve gastric emptying and in foals agonists that are commonly used to sedate horses.
  • 109. 108 EQUINE CLINICAL PHARMACOLOGYXylazine (0.5 mg/kg i.v.) alone had minimal effects increased sympathetic outflow, primarily medi-on gastroduodenal motility, but when combined ated through combined stimulation of Ci2 adreno-with the narcotic agonist-antagonist butorphanol ceptors and inhibition of acetylcholine release.there is pronounced suppression of antroduode- These external neural influences on intestinalnal myoelectric activity (Merritt et aI1998). Inter- motility are common targets for prokinetic drugs,estingly, butorphanol alone had minimal effects but events within the bowel can have importanton antroduodenal myoelectric activity. However, effects on intestinal motility and cause the boweldetomidine, at 0.0125 mg/kg, markedly depressed to be refractory to traditional prokinetic therapy.duodenal motility and would be expected to Release of cytokines from activated inflammatoryaffect gastric motility similarly. cells is probably an important feature of ileus in many cases. Ileus secondary to reperfusion injury is an anticipated response in horses with small intestinal obstruction. However, even apparentlySMALL AND LARGE INTESTINE mild intestinal injury can initiate cellular responses that lead to impaired motility. Mild intestinalPATHOPHYSIOLOGY OF insult by gentle surgical manipulation activatedINTESTINAL MOTILITY DISORDERS adhesion molecules on leukocytes and increasedDisorders that affect intestinal motility, including the expression of P-selectin and intercellular adhe-ileus, peritoneal inflammation, enteritis and sion molecule 1 on endothelial cells within theobstructive disorders, are frequent problems in vasculature of the muscularis layer of the intes-horses. Often many of these elements occur con- tine (Kalff et al 1999). Surgical manipulation ofcurrently. Gastrointestinal ileus is the functional the rodent small intestine resulted in substantialinhibition of propulsive bowel activity, irrespec- extravasation of leukocytes into the intestinal mus-tive of its pathophysiological basis. Ileus accom- cularis, consisting mainly of polymorphonuclearpanies a multitude of intestinal disorders, neutrophils, monocytes and mast cells and last-including strangulating and non-strangulating ing for days. This cellular inflammatory responseobstructions, mechanical obstruction with gas within the intestinal muscularis externa wasdistention, enteritis, endotoxemia and peritonitis. associated with a marked decrease in jejunal cir-There are many pathophysiological mechanisms cular muscle activity (Kalff et al 1998).resulting in disrupted propulsive intestinal motil-ity, many of which are unknown. Disruption ofthe primary neuromuscular pathways and the CLINICAL CONDITIONS WITHintestinal MMCs are well-characterized mecha- INTESTINAL MOTILITY DISORDERSnisms of ileus that result from several causes. Forinstance, subsequent to large colon volvulus, the Altered gastrointestinal motility can result fromneurons in the myenteric plexus undergo degen- many disorders, not all of which primarily affecteration and decrease in number (Schusser & White the gut. For instance, endotoxemia, regardless1997).Additionally, alterations in smooth muscle of its origin, can depress intestinal motility. Motil-mitochondrial morphology have been identi- ity is also probably affected by diet, subclinicalfied in equine jejunum after low-flow ischemia endoparasitism and even exercise.(Dabareiner et al 1995). In neonatal foals, enteritis is associated with Peritoneal inflammation or irritation can initi- the accumulation of fluid in the bowel lumen andate ileus, partly through spinal reflexes (Sjoqvist either poor propulsive motility or excessive con-et aI1985). Afferent fibers from peritoneal surfaces tractions of the bowel. Also, neonatal foals withterminate in the dorsal horn of the spinal cord, septicemia often have poor intestinal motility. Inwhere they can activate inhibitory sympathetic these cases, neither enteral feeding nor pharma-fibers or synapse directly onto sympathetic gan- cological stimulation of intestinal motility is indi-glia. The efferent limb of the reflex expresses cated. In older foals, duodenitis is associated with
  • 110. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 109delayed gastric emptying and the prokinetic drugs prefer to use a surgical approach to the manage-are useful in some cases. ment of cecal dysfunction. Cecal impactions often In adult horses with duodenitis/proximal require several days until there is complete reso-enteritis (OPE) there are inflammatory changes of lution and feeding can be resumed. Drugs thatvariable severity affecting all layers of the prox- alter intestinal motility, such as cholinomimetics,imal small intestine. Most affected horses pro- benzamides and motilides, may be useful in theduce copious volumes of enterogastric reflux and management of some cases of cecal impaction orit is presumed that impaired intestinal motility in postoperative management of these cases.allows the fluid to accumulate within the intes- Large-colon impaction is characterized by dis-tine and that pharmacological enhancement of tention of the large intestine with desiccatedintestinal motility will reduce the volume of digesta. All segments of the large colon can bereflux. In most cases, use of prokinetic drugs does involved but the pelvic flexure and right dorsalnot appear to be associated with a reduction in colon are the most frequent sites of impaction.enterogastric reflux. This may reflect an inability Impactions of the large colon are probably a com-of these drugs to stimulate smooth muscle con- bination of motility and fluid-balance disorderstraction in the inflamed intestine, overriding ENS and most cases are treated with laxatives or hydra-stimuli that cannot be impacted appreciably by the tion solutions.current roster of prokinetics, and incorrect target- Impaction of the small colon with meconiuming of pathophysiological processes. occurs in neonatal foals. Typically, these impac- POI is a frequent complication of abdominal tions result from an excessive amount of meco-surgery in horses. In most cases, the small intestine nium. Small-colon impactions are unusual in olderis the segment of bowel affected primarily and foals and occur infrequently in adult horses.strangulation and distension are the most frequent Apparently there is a positive association betweenproblems. In both instances, the blood flow to all small-colon impaction and fecal culture oflayers of the intestine is compromised and reper- Salmonella spp. (Rhoads et al 1999). Small-colonfusion injury is a feature of the postoperative impactions in adult horses may represent aperiod. Reperfusion injury is a complex process primary motility disorder secondary to inflam-involving mitochondrial and cytosolic changes, mation. Small-colon impactions are often treatedactivation of a myriad of inflammatory media- conservatively using enemas, although in sometors and increased endothelial cell permeability cases surgery is required. Drugs that alter motil-and neutrophil recruitment. Changes can occur ity are typically not used in these cases.throughout much of the intestine, even away fromthe primary site of injury and the magnitude of DRUGS ACTING ON MOTILITY OFinjury is often difficult to predict. As mentioned THE SMALL AND LARGE INTESTINEabove, simple surgical manipulation of the intes-tine can promote neutrophil recruitment within the Cholinomimeticsintestine with subsequent impairment of propul- Bethanecholsive motility (Kalff et aI1998). Clinical manifesta-tions of POI can include persistent enterogastric Bethanechol increased electrical activity in thereflux, lack of borborygmi and inability to toler- small intestine (Roger & Ruckebusch 1987) andate feeding, for up to 3-5 days. i.v. administration (0.025mg/kg) was shown to Cecal dysfunction and cecal impaction occur rapidly initiate MMC phase III activity in theinfrequently but can be very difficult to manage. ileum of horses, although the effects on transit ofMany clinicians consider these to be separate digesta through the small intestine were notentities, with cecal dysfunction characterized by determined (Lester et al 1998a). Bethanechol alsocecal distention with soft to watery contents and increased the rate of emptying of radiolabeledcecal impaction characterized by accumulation markers from the cecum, which was associatedof doughy, desiccated ingesta. Many clinicians with an increase in the relative strength and
  • 111. 110 EQUINE CLINICAL PHARMACOLOGYduration of contractions in the cecum and right examination of the rectum may be facilitated byventral colon. Bethanechol is not used routinely administration of an anticholinergic drug.for treating impactions of the cecum and largecolon and its effects may be different in these Atropinecases than in normal horses. Atropine is a potent inhibitor of intestinal motility (Adams et a11984, Roger & Ruckebusch 1987). ItNeostigmine is most frequently applied topically to the cornea to promote mydriasis and anecdotal reports sug-Neostigmine increases receptor levels of acetyl- gest that the absorption of atropine from thischoline by inhibiting the enzyme cholinesterase. site may disrupt intestinal motility (see Ch. 13).Neostigmine (0.02mg/kg i.v.) increased myo- A single s.c, dose of atropine (0.02mg/kg) canelectrical activity in the jejunum but significantly be given safely to most horses. Dose rates ofdelayed the emptying of 6 mm beads from the 0.02-0.04 mg/kg may facilitate evacuation of thestomach of normal adult horses (Adams & rectum in horses with a rectal tear.MacHarg 1985). The i.v, infusion of neostigminewas associated with signs of abdominal discom- Scopolamine (hyoscine)fort. In another report, the i.v, administration ofneostigmine (0.025mg/kg) promoted cecal and Scopolamine (hyoscine N-butylbromide) is avail-colonic contractile activity and hastened the emp- able as an antispasmodic product (20 mg ampuletying of radiolabeled markers from the cecum but for injection and 20 mg tablets) for use inalso induced defecation and caused mild abdom- humans. In addition, there is a veterinary prod-inal pain (Lester et a11998a). Neostigmine is used uct that contains scopolamine and dipyroneby some clinicians to counteract POI and to pro- approved for use as an antispasmodic/ analgesicmote emptying of the cecum or large colon in cases in animals. In addition to blocking the effects ofof impaction. As a cautionary note, it has been sug- acetylcholine at the muscarinic receptor, scopo-gested that, because of the force of neostigmine- lamine affects nicotinic receptors in intestinalinduced activity in the cecum, the drug ganglia and does not affect the CNS. In horses,should probably be avoided in horses with scopolamine is used as an antispasmodic and toimpaction or large intestinal distension (Lester relax the rectum to facilitate abdominal palpa-et a11998a). Other drugs, such as bethanechol or tion. In one report, 0.2 mg/kg was as effective aserythromycin, appear to be as effective or more 0.2 mg/kg scopolamine plus 2.5mg/kg dipyroneeffective in promoting gastrointestinal transit and in relieving discomfort caused by balloon dilata-are associated with fewer adverse effects. tion of the cecum (Roelvink et a11991). The anal- gesic effect lasted for 20 min.Anticholinergic drugs BenzamidesAnticholinergic drugs compete with acetylcholine Metoclopramidefor muscarinic binding sites on smooth muscle,thereby decreasing smooth muscle activity and There are conflicting data on the use of metoclo-intestinal motility. In most cases, anticholinergic pramide in horses. In some models of POI, meto-effects in the intestine are undesirable, often result- clopramide appeared effective (Gerring & Hunting in desiccation and accumulation of digesta 1986), while others found that metoclopramidewithin the large intestine (impaction). A useful did not positively affect intestinal motility (Sojkaapplication of anticholinergic drugs is to relax the et al 1988). Metoclopramide crosses the blood-rectum, which may be desirable to facilitate pal- brain barrier, where its antagonist properties onpation of the abdomen per rectum or in rectal tears. central dopamine O2 receptors can result inIn the latter, the evacuation of rectal contents and extrapyramidal signs, including violent excitation.
  • 112. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 111These effects, seen with i.v. doses of 0.25mg/kg the intestinal tract and the expense of the par-and higher, have resulted in poor acceptance of enteral formulation. Cisapride has been with-the drug in equine practice. Recently, the use of a drawn from the market in the USA and Europe.low-dose, constant infusion of metoclopramidewas examined in 70 horses undergoing smallintestinal resection (Dart et aI1996). Constant i.v. Motilidesinfusion of metoclopramide (0.04mg/h per kg) Erythromycin lactobionate has been shown tosignificantly decreased the volume and duration promote cecal emptying in normal horses whenof gastric reflux postoperatively over that seen in given as an i.v, bolus dose at 0.1 mg/kg or as aboth control and intermittent drug infusion in 60min i.v, infusion at 1.0mg/kg (Lester et althese clinical cases. The study was not designed 1998b). The maximal effect on emptying was seenin such a way that allows definitive conclusions at 1.0mg/kg and was greater than that induced byto be drawn on the efficacy of the treatment; how- neostigmine or bethanechol. Erythromycin had noever, the infusion was well tolerated. effect on ileal myoelectric recordings but induced premature MMC phase III regular spiking activityCisapride in the cecum and increased myoelectric activity in the right ventral colon. There are no experimentalInterest in the use of cisapride to improve intestinal data on the effects of erythromycin on cecal emp-motility in horses arose from its use to affect gas- tying under clinical conditions, but clinical impres-trointestinal motility positively without producing sions support its use (i.v, bolus dose of O.5mg/kgthe side-effects associated with other prokinetic every 8 h) in the prevention and treatment of cecaldrugs in humans and other animals. One of the impaction. Erythromycin potentially inducesbenefits of cisapride over other prokinetic drugs clostridial colitis and this must be considered ifis its effect on promoting motility in the colon. erythromycin is to be used in adult horses.Early studies showed that cisapride administeredi.v. (0.05mg/kg) to horses produced marked andprolonged increases in electrical and mechanical Sympathomimeticsactivity in several bowel segments (King & Gerring <X2 adrenergic agonists1988). In the stomach, there was an increase intotal contractile activity with increased contraction Xylazine and detomidine are <X2 adrenoceptoramplitude and a slight reduction in contraction agonists that are used frequently in horses withrate. In the small intestine, there was an increase colic. They provide temporary analgesia but alsoin MMC phase II activity with an increase in the have cardiovascular effects that may be counter-number and amplitude of contractions and a productive in horses with already compromiseddecrease in phase III activity. Cisapride increased cardiovascular function. Xylazine was reportedelectrical and contractile activity in the left dorsal to induce phase III spiking activity and reset thecolon with increased contraction amplitude and MMC cycle in the duodenum of horses (Merrittan increase in electrical activity in the small colon. et a11989) and to increase regular spiking activityIn a report on experimental and clinical use, it in the jejunum (Adams et al 1984). The effect ofwas suggested that the i.m. administration of xylazine on the coordinated propulsion of ingesta0.1mg/kg cisapride was effective in preventing through the small intestine is not known and mayor minimizing POI (Gerring & King 1989). not be correlated with these myoelectric findings. In the USA, cisapride was only available as an Xylazine and detomidine have been shown tooral formulation, which was poorly absorbed decrease cecal motility and blood flow and may bewhen given by intragastric administration (Steel risk factors for anesthetic-associated cecal impac-et al 1998) or per rectum (Cook et al 1997, Steel tion. The <X2 adrenoceptor agonists depressed myo-et aI1999). The use of cisapride in horses has been electric activity and decreased the cecal emptyinglimited by the erratic absorption of the drug from rate and Tso of emptying radiolabeled markers
  • 113. 112 EQUINE CLINICAL PHARMACOLOGYfrom the cecum (Lester et al 1998a, Rutkowski and increase sympathetic nerve activity, con-et aI1991). In ponies in which pelvic flexure fistu- tributing to intestinal ileus. Yohimbine is an U2las were created and inflation of an intraluminal adrenoceptor antagonist that has been shown toballoon was used to simulate impaction, xylazine affect the equine small intestine and colon (Lesterrelieved clinical signs of colic for 30 to 60 min and et aI1998a). In normal horses, yohimbine mildlyreduced intraluminal pressure in the pelvic flex- accelerated cecal emptying. In a serosal traumaure (Lowe et aI1980). However, xylazine was also model of POI, yohimbine restored some electrical-associated with decreased mean arterial blood mechanical function to the small intestineflow through the colic arteries. (Gerring & Hunt 1986). In an equine endotox- Clearly, the U2 adrenergic agonists have both emia model, slow i.v, infusion of 0.075mg/kgbenefits and risks. In horses with impaction of yohimbine attenuated the suppression of thethe large colon, they can relax the colon and pro- mechanical activity and blood flow in the cecumvide pain relief but will also delay the transit of and large colon (Eades & Moore 1993). The clinicaldigesta. They are probably best utilized early in utility of yohimbine to promote intestinal transittreatment, when pain from distention is a prob- has not been examined and this drug does notlem but the propulsion of the desiccated digesta appear to be used extensively.through the cecum and colon is unlikely until thedigesta are better hydrated. Use of the these Lidocaine (lignocaine)drugs in horses with more severe intestinal disor-ders and compromised cardiovascular function The i.v, administration of lidocaine has beenshould probably be accompanied by supportive suggested as a treatment that restores intestinaltreatment (fluid therapy). motility in horses with POI. Lidocaine has been evaluated in both spontaneous and experimental132 adrenergic agonists models of intestinal ileus. It does not stimulate bowel motility directly but appears to prevent theThe 132 adrenoceptor agonists, such as clenbuterol, inhibition of bowel motility under certain condi-are used to treat respiratory disorders in horses tions. In rats, epidural administration of lidocainebut they also may impact on gastrointestinal func- was associated with a significant recovery intion since they decrease colonic motility (Lyrenas intestinal motility in an intestinal ischemia model1985). The 132 adrenoceptor agonists induced sig- (Udassin et al 1994). In humans, i.v. administra-nificant decreases in the amplitude and frequency tion of lidocaine reduced POI after non-intestinalof contraction in equine ileal smooth muscle abdominal surgery (Rimback et aI1990). This effectin vitro (Malone et al 1996). The effects of thera- appears to be mediated by the suppression ofpeutic doses of 132 adrenergic agonists used for primary afferent neurons from the bowel to spinalthe treatment of respiratory disease on alimen- segments, thereby limiting reflex sympathetictary function in horses is not known, but the efferent inhibition of motility. In addition, localpotential for inhibition of intestinal motility anesthetics have anti-inflammatory properties,should be considered, particularly when adminis- including inhibiting both granulocyte migrationtering 132 adrenergic agonists parenterally. and the release of lysosomal enzymes. In models of myocardial and lung ischemia-reperfusionSympatholytics injury, lidocaine significantly reduced neutrophil recruitment and tissue injury (Vitola et aI1997).Yohimbine The beneficial effects of i.v. lidocaine in horsesActivation of presynaptic U2 adrenoceptors inhibits with POI have not been confirmed, although thethe release of acetylcholine from cholinergic neu- results of a multicenter study suggested that par-rons, resulting in decreased muscular contractions. enteral lidocaine may help to restore intestinalDisorders such as endotoxemia and peritonitis motility in horses with POI (Malone et al 1998).elicit the release of sympathetic neurotransmitters An initial loading dose of 1.3 mg/kg is given
  • 114. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 113followed by a constant i. v. infusion at 0.05mg/ result, in part, from insufficient secretion of fluidmin per kg. Lidocaine infusion is associated into the large intestine and desiccation of thewith reversible side-effects including muscle fas- digesta. Frequently diagnosed causes of diarrheaciculations, ataxia and seizures. in foals include infections with rotavirus, Clostridium difficile, C. perfringens and Salmonella spp. Less- frequent causes of diarrhea in foalsPATHOPHYSIOLOGY OF include E. coli, Rhodococcus equi and endopara-INTESTINAL SECRETORY sites. Frequently diagnosed causes of diarrhea inDISORDERS adult horses include Salmonella spp., C. difficile,Disorders that cause increased secretion of fluid C. perfringens and Ehrlichia risticii. However, theand electrolytes into the small intestine of the horse cause of diarrhea in most foals and horses is usu-are characterized by abdominal discomfort, disten- ally not determined. Mechanisms of fluid secretionsion of the small intestine and enterogastric reflux. include enterotoxin-induced secretion mediated byIn young foals with small intestinal secretory dis- cyclic nucleotides, mast cell degranulation, cyto-orders, diarrhea may occur. Increased intestinal toxiceffects and increased tissue permeability fromsecretion can result from the active secretion of damage by the products of inflammatory cells.electrolytes and water, for example the cyclic Most digesta impactions in foals and horsesnucleotide-stimulated secretion that results from occur in the large intestine and small colon. Typi-exposure to bacterial enterotoxins. Passive secre- cally an increased volume of desiccated digestation of water can result from increased permeabil- characterizes impactions of the cecum. Otherity of the intestine, such as in enteritis, distension forms of cecal dysfunction are characterized byor ischemia, or decreased absorption of osmoti- digesta with a normal to increased proportion ofcally active substances, such as with lactose intoler- fluid. The former condition may be a combina-ance in foals. Disorders in which there is decreased tion of disorders of motility and fluid fluxessecretion of fluid into the small intestine are not while the latter is presumed to represent a motil-appreciated, although impactions of ingesta in ity disorder. Impactions of the large colon aresegments of the small intestine can occur. typically located in the left ventral colon, pelvic Horses with OPE can produce copious volumes flexure and right dorsal colon. They are charac-of enterogastric reflux. It is generally presumed terized by an increased mass of desiccated digesta.that this fluid originates as secretion from the Drugs that decrease the motility of the large colon,inflamed small intestinal mucosa, either as an such as atropine and amitraz, can cause large-active, cyclic nucleotide-mediated fluid secretion colon impactions, but disruptions in fluid fluxesor as an exudative secretion. Additionally, enteric during fermentation of feed may be more rele-nerve-mediated secretion, via mast cell degranula- vant in clinical cases.tion, may contribute to the fluid secretion. Anotherpossible source of the large volume of watery DRUGS ACTING ON INTESTINALsecretions is the pancreas. Endoscopy of a small SECRETION AND ABSORPTIONnumber of horses with OPE has revealed an almostcontinuous stream of fluid emanating from the Laxativesmajor duodenal papilla. Normally, much of the Lubricantsfluid secreted through the major duodenal papilla,up to II/h, enters the stomach and it is logical to Mineral oil is used frequently in equines withpresume that increased pancreatic secretion will large intestine impaction to lubricate the impactedresult in increased enterogastric reflux. digesta and facilitate its passage through the Disorders that cause increased secretion of colon. Its effectiveness is limited for large, desic-fluid and electrolytes in the large intestine of the cated impactions because the oil tends to passhorse are characterized by diarrhea. Conversely, around the impaction. Therefore, for mineral oil toimpactions of ingesta in the large intestine may be most effective, the digesta must be adequately
  • 115. 114 EQUINE CLINICAL PHARMACOLOGYhydrated. Linseed oil has been compared with chemical cathartics, such as castor oil, alsomineral oil in normal horses; whereas the admin- can cause intestinal mucosal injury. Castor oil isistration of mineral oil softened manure and was used traditionally as a laxative in humans. Thebenign to the horse, linseed oil caused more pro- active ingredient in castor oil is ricolinoleate. Thelonged stool softening but produced signs of mechanism of the laxative effect of castor oil mayabdominal discomfort in some horses and was involve an intrinsic afferent reflex, with theassociated with increased serum glucose and afferent limb mediated by tachykinins and withbilirubin concentrations (Schumacher et aI1997). NO as the efferent stimulus to secretion. In addi- tion to inducing fluid secretion, castor oil canOsmotic cathartics induce substantial mucosal damage (Johnson et al 1993). A dose of 60ml (1 ml/kg) p.o. has beenOsmotic cathartics are hypertonic solutions that mentioned for treatment of meconium impactionare absorbed poorly from the lumen of the intes- in foals (Madigan 1997). It should be noted thattine and draw water into the intestine by passive severe colitis has been induced in horses by thediffusion. The small intestine has a relatively low administration of 2.5 ml/kg castor oil (Robertsdensity of intercellular tight junctions and con- et al1989).sequently should be most responsive to theseosmotic agents. The large intestine, a target forosmotic cathartics and a frequent site of impactions Surfactantsof digesta, has a relatively high density of inter- Dioctyl sodium sulfosuccinate (DSS) is thoughtcellular tight junctions and here the osmotic to be useful in resolving impactions of the largecathartics should be expected to have less effect. intestine by acting as a surfactant and facilitating The osmotic cathartics used frequently in the movement of water into desiccated digesta inequine veterinary practice are magnesium sulfate the colon. In one study, DSS given at 50mg/kgand magnesium oxide. The use of these agents has in 61 of water to four normal horses resultedbeen based on the presumption that they induce in increased fecal output and fecal water contenta passive secretion of water into the intestine from 6 to 12h after administration (Freeman et althrough their osmotic properties. However, this 1992). DSS caused mild colic, hyperpnea andeffect may be minimal. Magnesium is absorbed diarrhea in one horse 0.3-3 h after administration.in the intestine and magnesium toxicosis has been There are many anecdotal reports of DSS toxi-reported following the administration of magne- cosis, which has clinical signs similar to carbohy-sium sulfate (Henninger & Horst 1997). Also, in drate overload (signs of endotoxemia, diarrheaone report, magnesium sulfate, given at 1.0 g/kg, and laminitis). DSS is available as a 5% andincreased fecal output and fecal water content 10% solution and the dose should not exceedbut in a timeframe that was too short to be 25mg/kg (8.3 ounces (USA) or 250 ml of a 5%explained by an osmotic effect in the large intes- solution to a 500 kg horse).tine (Freeman et aI1992). An effect of magnesiumsulfate was seen by 5 h after administration andpreceded the appearance of a concurrently admin- Fiberistered liquid transit marker in the feces by many Fiber is popularly considered to have a laxativehours, suggesting that this was not a direct osmotic effect and, indeed, in carnivores and omnivoreseffect but was a result of the secretion of water by this can be true. In hindgut fermenting herbivores,the large intestine or small colon giving rise to such as the horse, it seems unlikely that fiber, suchincreased fecal water content and output. as that found in bran, will increase fecal output or water content. Another use of fiber as a laxative isChemical cathartics in the management of accumulation of sand inChemical cathartics stimulate intestinal fluid the large intestine. In one study, psyllium admin-secretion by activating mucosal secretion. Some istration had no effect on removal of sand from the
  • 116. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 115large intestine in ponies in which sand was a study on the effect of BSSin humans with histo-placed into the cecum (Hammock et aI1998). logic colitis, patients ingested eight 262mg tablets per day. Each tablespoon (15ml) of the BSS sus- pension available contains 262 mg BSS. An effec-Antisecretory agents tive dose for horses has not been determined.True antisecretory agents, those that block a bio- Empirical doses of 30 ml every 2-4 h have beenchemical process that promotes intestinal fluid used in foals. Larger volumes, up to 500 ml everysecretion, are not available for clinical use in 4 h, have been administered to adult horses withhorses. Loperamide can reduce the volume of diar- diarrhea. BSS is generally safe (Tillman et alrhea in foals with a primarily small intestinal secre- 1996), although adverse effects can include con-tory disorder. However, treated foals may become stipation and it may interfere with intestinalcolicky as a result of fluid distention in the intes- absorption of some drugs (Ericsson et al 1982).tines because the mechanism of action is primarilyretention of fluid within the intestine. Also, reten-tion of intestinal content may promote the proli- Prostaglandin analogsferation of enteropathogens. The enkephalinase Right dorsal colitis is a syndrome that results frominhibitor racecadotril appears to have true antise- excessive NSAID administration in horses; it iscretory effect in animal models and in humans characterized by ulceration of the mucosa of thewith diarrhea (Izzo et aI1998). Its safety or effec- right dorsal colon, plasma protein loss and weighttiveness in foals and horses has not been reported. loss. Treatment with misoprostol, a prostaglandin E1 analog has been proposed, but not proven, toAGENTS PROMOTING DIGESTION result in a positive clinical response in affectedAND ABSORPTION horses. In one report, coadministration of phenyl- butazone with a prostaglandin E2 analog signifi-Lactase-replacement therapy cantly attenuated the alimentary mucosal injuryLactose intolerance is an infrequent primary dis- compared with phenylbutazone administrationorder in foals that typically manifests as diarrhea alone (Collins & Tyler 1985). Recommendedand intermittent abdominal discomfort. Lactose dosages of misoprostol range from 1.5 to 5l-Lg/kgintolerance is more frequently seen in young p.o. three times a day. In clinical cases, the authorfoals with enteritis. In these foals, ingestion of typically begins treatment at 1.5l-Lg/kg p.o. threemilk or milk replacer containing lactose is associ- times a day and gradually increases this up toated with diarrhea and abdominal discomfort. 2.5-3.0 I-Lg/ kg, because higher doses have beenThe concurrent administration of lactase (one associated with abdominal discomfort and diar-tablet per hour) concurrently with nursing or rhea in clinical cases. Treatment usually lasts forfeeding a milk replacer appears to be beneficial 4-5 weeks.until the foals intestinal mucosa heals andendogenous production of lactase becomes suffi-cient. Different lactase products are available in ANTIMICROBIAL AGENTSEurope. Antimicrobial agents are used frequently in horsesDRUGS ACTING ON INTESTINAL with gastrointestinal disorders but in most casesMUCOSA their use is empirical. For some infectious disor- ders of the alimentary tract, there are specific anti-Bismuth subsalicylate microbial treatments, including:There are no studies on the effect of bismuth com- • clostridiosis (c. difficile, C. perfringens):pounds on diarrhea in foals or horses, although metronidazole 15 mg/kg p.o. three timessuch products are used frequently in practice. In a day;
  • 117. 118 EQUINE CLINICAL PHARMACOLOGY• equine ehrlichial colitis (E. risticii): oxytetra- Botella A. Vabre F. Fioramonti J et al1993 In vivo inhibitory effect of lanreotide (BIM 23014), a new somatostatin cycline llmg/kg i.v., twice daily; analog, on prostaglandin- and cholera toxin-stimulated• pylogranulomatous enteritis (R. equi): intestinal fluid in the rat. Peptides 14:297-301 erythromycin phosphate 35mg/kg p.o. and Brzozowski T, Konturek S J. Sliwowski Z et al1997 Role of L-arginine, a substrate for NO-synthase, in rifampin 10mg/kg p.o. both twice a day; gastroprotection and ulcer healing. Journal of• proliferative enteropathy (Lawsonia intraceliu- Gastroenterology 32:442-452 laris): erythromycin phosphate 35 mg/kg p.o. Camilleri M 2001 Tegaserod. Alimentary Pharmacology and Therapeutics 15:277-289 twice a day; and Campbell-Thompson M L, Merritt A M 1987 Effect of• candidiasis (Candida albicans): fluconazole ranitidine on gastric acid secretion in young male 8 mg/kg p.o. three times a day. horses. American Journal of Veterinary Research 48:1511-1515The gastrointestinal tract is a frequent site for Campbell-Thompson M L, Merritt A M 1990 Basal and pentagastrin-stimulated gastric secretion in youngadverse effects of antimicrobial drugs, primarily horses. American Journal of Physiologybecause of disruption of normal intestinal micro- 259:R1259-R1266bial populations and proliferation of enteropatho- Cederberg C, Andersson T. Skanberg 11989 Omeprazole: pharmacokinetics and metabolism in man. Scandinaviangens. Diarrhea, often with accompanying signs of Journal of Gastroenterology Supplement 166:33-42endotoxemia, is the usual clinical manifestation. Clark C K, Merritt A M, Burrow J A et al1996 Effect of anAntimicrobial agents known to be, or implicated aluminum-magnesium hydroxide antacid and BSS on gastric pH in horses. Journal of the American Veterinaryin being, associated with antimicrobial-induced Medical Association 208:1687-1691diarrhea include penicillin, ceftiofur, lincomycin, Clarke L L, Argenzio R A, Roberts M C 1990 Effect of mealtetracycline, erythromycin and the potentiated feeding on plasma volume and urinary electrolyte clearance in ponies. American Journal of Veterinarysulfonamides. Erythromycin can also promote Research 51:571-576diarrhea via its motilide activity. Collins L G. Tyler D E 1985 Experimentally induced phenylbutazone toxicosis in ponies: description of the syndrome and its prevention with syntheticREFERENCES prostaglandin E2 . 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  • 119. 118 EQUINE CLINICAL PHARMACOLOGYJenkins C, Frazier D, Blackford J et al 1992a Duration of Lester G D, Merritt A M, Neuwirth Let al 1998b Effect of anti-secretory effects of omeprazole in horses with erythromycin lactobionate on myoelectric activity of chronic gastric cannulae. Equine Veterinary Journal ileum, cecum and right ventral colon and cecal Supplement 13:89-92 emptying of radio labeled markers in clinically normalJenkins C C, Frazier D L, Blackford J T et al 1992b ponies. American Journal of Veterinary Research Pharmacokinetics and anti-secretory effects of 59:328-334 intravenous omeprazole in horses. Equine Veterinary Lippold B S, Weiss R, Mevissen M et al 2002 The properties Journal Supplement 13:84-88 of a new promotile drug, tegaserod (HTF 919) inJohnson C M, Cullen J M, Roberts M C 1993 Morphologic equines. In: Proceedings of the 7th International Equine characterization of castor oil-induced colitis in ponies. Colic Research Symposium, Birmingham, UK, p. 46 Veterinary Pathology 30:248-255 Longo W E, Vernava A M D 1993 Prokinetic agents forKalff J C, Schraut W H, Simmons R L et al 1998 Surgical lower gastrointestinal motility disorders. Diseases of the manipulation of the gut elicits an intestinal muscularis Colon and Rectum 36:696-708 inflammatory response resulting in postsurgical ileus. Lowe J E, Sellers A F, Brondum J 1980 Equine pelvic Annals of Surgery 228:652-663 flexure impaction. A model used to evaluate motorKalff J C, Carlos T M, Schraut WHet al1999 Surgically events and compare drug response. Cornell Veterinarian induced leukocytic infiltrates within the rat intestinal 70:401-412 muscularis mediate postoperative ileus. Lyrenas E 1985 Beta adrenergic influence on esophageal Gastroenterology 117:378-387 and colonic motility in man. Scandinavian Journal ofKalff J C, Schraut W H, Billiar T R et al 2000 Role of Gastroenterology Supplement 116:1-48 inducible NO synthase in postoperative intestinal MacAllister C G, Lowrey F, Stebbins M et al 1994 smooth muscle dysfunction in rodents. Transendoscopic electrocautery-induced gastric ulcers Gastroenterology 118:316-327 as a model for gastric healing studies in ponies. EquineKamerling S G, Hamra J G, Bagwell C A 1990 Naloxone- Veterinary Journal 26:100-103 induced abdominal distress in the horse. Equine MacAllister C G, Sangiah S 1993 Effect of ranitidine on Veterinary Journal 22:241-243 healing of experimentally induced gastric ulcers inKatschinski M, Steinicke C, Reinshagen M et al 1995 ponies. American Journal of Veterinary Research Gastrointestinal motor and secretory responses to 54:1103-1107 cholinergic stimulation in humans. Differential MacDonald T M 1991 Metoclopramide, domperidone and modulation by muscarinic and cholecystokinin receptor dopamine in man: actions and interactions. European blockade. European Journal of Clinical Investigation Journal of Clinical Pharmacology 40:225-230 25:113-122 Madigan J E 1997 Disorders of the first two weeks ofKatz J 1991 Acid secretion and suppression. Medical Age. In: Madigan J E (ed.) Manual of equine neonatal Clinics of North America 75:877-887 medicine, 3rd edn. Live Oak Publishing, Woodland,King J N, Gerring E L 1988 Actions of the novel CA, p. 119 gastrointestinal prokinetic agent cisapride on equine Malone E D, Brown D R, Trent A M et al 1996 Influence of bowel motility. Journal of Veterinary Pharmacology adrenergic and cholinergic mediators on the equine and Therapeutics 11:314-321 jejunum in vitro. American Journal of VeterinaryKing J N, Davies J V, Gerring E L 1990 Contrast Research 57:884-890 radiography of the equine oesophagus: effect of Malone E D, Turner T A, Wilson J H 1998 Intravenous spasmolytic agents and passage of a nasogastric tube. lidocaine for the treatment of equine ileus. In: Equine Veterinary Journal 22:133-135 Proceedings of the 6th Equine Colic ResearchKitchen D L, Merritt A M, Burrow J A 1998a Histamine- Symposium, Athens, GA, p. 42 induced gastric acid secretion in horses. American Matheson A J, Noble S 2000 Racecadotril. Drugs Journal of Veterinary Research 59:1303-1306 59:829-837Kitchen D L, Merritt A M, Burrow J A et al1998b Source Mathews C J, MacLeod R J, Zheng S X et al1999 of non-parietal component of pentagastrin-stimulated Characterization of the inhibitory effect of boiled rice fasting equine gastric contents. In: Proceedings of the on intestinal chloride secretion in guinea pig crypt 6th Equine Colic Research Symposium, Athens, GA, cells. Gastroenterology 116:1342-1347 p. 35 McCallum R W 1999 Pharmacologic modulation of motility.Konturek S J, Brzozowski T, Majka J et al 1992 NO in Yale Journal of Biology and Medicine 72:173-180 gastroprotection by aluminum-containing antacids. McCarthy D M 1991 Sucralfate. New England Journal European Journal of Pharmacology 229:155-162 of Medicine 325:1017-1025Krejs G J 1986 Physiological role of somatostatin in the Merritt A M 1999 Normal equine gastroduodenal secretion digestive tract: gastric acid secretion, intestinal and motility. Equine Veterinary Journal Supplement absorption and motility. Scandinavian Journal of 29:7-13 Gastroenterology Supplement 119:47-53 Merritt A M, Burrow J A, Horbal M J et al 1996 EffectLester G D, Merritt A M, Neuwirth Let al1998a Effect of of omeprazole on sodium and potassium output in alpha 2 adrenergic, cholinergic and nonsteroidal anti- pentagastrin-stimulated equine gastric contents. inflammatory drugs on myoelectric activity of ileum, American Journal of Veterinary Research 57:1640-1644 cecum and right ventral colon and on cecal Merritt A M, Burrow J A, Hartless C S 1998 Effect of emptying of radiolabeled markers in clinically normal xylazine, detomidine and a combination of xylazine and ponies. American Journal of Veterinary Research butorphanol on equine duodenal motility. American 59:320-327 Journal of Veterinary Research 59:619-623
  • 120. 6. DRUGS ACTING ON THE GASTROINTESTINAL SYSTEM 119Merritt A M, Campbell-Thompson M L, Lowrey S 1989 Rimback G, Cassuto J, Tolle5son P 01990 Treatment of Effect of xylazine treatment on equine proximal postoperative paralytic ileus by intravenous lidocaine gastrointestinal tract myoelectrical activity. American infusion. Anesthesia and Analgesia 70:414-419 Journal of Veterinary Research 50:945-949 Ringger N C, Lester G 0, Neuwirth L et al 1996 Effect ofMerritt A M, Sanchez L S, Burrow J A et al 2002 bethanechol or erythromycin on gastric emptying in Bioavailability of "Gastroqardlv" vs. three generic horses. American Journal of Veterinary Research compounded omeprazole preparations in mature 57:1771-1775 horses. In: Proceedings of the 7th International Equine Roberts M C, Clarke L L, Johnson C M 1989 Castor-oil Colic Research Symposium, Birmingham, UK, p. 81 induced diarrhoea in ponies: a model for acute colitis.Meyer G A, Rashmir-Raven A, Helms R J et al 2000 The Equine Veterinary Journal Supplement 7:60--67 effect of oxytocin on contractility of the equine Roelvink M E, Goossens L, Kalsbeek H C et al 1991 oesophagus: a potential treatment for oesophageal Analgesic and spasmolytic effects of dipyrone, obstruction. Equine Veterinary Journal 32:151-155 hyoscine-N-butylbromide and a combination of the twoMullersman G, Gotz V P, Russell W L, Derendorf H 1986 in ponies. Veterinary Record 129:378-380 Lack of clinically significant in vitro and in vivo Roger T, Bardon T, Ruckebusch Y 1985 Colonic motor interactions between ranitidine and sucralfate. Journal responses in the pony: relevance of colonic stimulation of Pharmaceutical Sciences 75:995-998 by opiate antagonists. American Journal of VeterinaryMurray M J, Eichorn E S 1996 Effects of intermittent feed Research 46:31-35 deprivation, intermittent feed deprivation with ranitidine Roger T, Ruckebusch Y 1987 Pharmacological modulation administration and stall confinement with ad libitum of postprandial colonic motor activity in the pony. access to hay on gastric ulceration in horses. American Journal of Veterinary Pharmacology and Therapeutics Journal of Veterinary Research 57:1599-1603 10:273-282Murray M J, Grodinsky C 1992 The effects of famotidine, Roth S H 1990 Misoprostol in the prevention of NSAID- ranitidine and magnesium hydroxide/aluminum induced gastric ulcer: a multicenter, double-blind, hydroxide on gastric fluid pH in adult horses. Equine placebo-controlled trial. Journal of Rheumatology Veterinary Journal Supplement 11:52-55 Supplement 20:20--24Murray M J, Nout Y S, Ward 0 L 2001 Endoscopic findings Ruppin H 1987 Loperamide: a potent antidiarrhoeal drug of the gastric antrum and pylorus in horses: 162 cases with actions along the alimentary tract. Alimentary (1996-2000). Journal of Veterinary Internal Medicine Pharmacology and Therapeutics 1:179-190 14:401-406 Rutkowski J A, Eades S C, Moore J N 1991 Effects ofMurray M J, Schusser G F 1993 Measurement of 24-h xylazine butorphanol on cecal arterial blood flow, cecal gastric pH using an indwelling pH electrode in horses mechanical activity and systemic hemodynamics in unfed, fed and treated with ranitidine. Equine Veterinary horses. American Journal of Veterinary Research Journal 25:417-421 52:1153-1158Ness T J 1999 Kappa opioid receptor agonists differentially Sams R A, Gerken 0 F, Dyke T M et al 1997 inhibit two classes of rat spinal neurons excited by Pharmacokinetics of intravenous and intragastric colorectal distention. Gastroenterology 117:388-394 cimetidine in horses. I. Effects of intravenous cimetidineNguyen A, Camilleri M, Kost L J et al 1997 SDZ HTF 919 on pharmacokinetics of intravenous phenylbutazone. stimulates canine colonic motility and transit in vivo. Journal of Veterinary Pharmacology and Therapeutics Journal of Pharmacology and Experimental Therapeutics 20:355-361 280:1270--1276 Sanchez L C, Lester G 0, Merritt A M 1998 Effect ofNieto J E, Spier S J, van Hoogmoed Let al 2001 ranitidine on intragastric pH in clinically normal neonatal Comparison of omeprazole and cimetidine in healing of foals. Journal of the American Veterinary Medical gastric ulcers and prevention of recurrence in horses. Association 212:1407-1412 Equine Veterinary Education 13:260--264 Sandin A, Andrews F M, Nadeau J A et al1999 Effects ofNwokolo C U, Mistry P, Pounder R E 1990 The absorption intramuscular omeprazole on gastric acid secretion in of bismuth and salicylate from oral doses of Pepto- horses over a twenty-four hour period. Equine Veterinary Bismol (bismuth salicylate). Alimentary Pharmacology Journal Supplement 29:50--53 and Therapeutics 4:163-169 Sandin A, Andrews F M, Nadeau J A et al 2000 Effect ofRabon E C, Reuben M A 1990 The mechanism and nervous excitation on acid secretion in horses. Acta structure of the gastric H+,K+-ATPase. Annual Review of Physiologica Scandinavica 168:437-442 Physiology 52:321-344 Sangiah S, MacAllister C C, Amouzadeh H R 1988 EffectsRedmond L M, Cross 0 L, Strickland J R et al 1994 Efficacy of cimetidine and ranitidine on basal gastric pH, free of domperidone and sulpiride as treatments for fescue and total acid contents in horses. Research in toxicosis in horses. American Journal of Veterinary Veterinary Science 45:291-295 Research 55:722-729 Sangiah S, MacAllister C C, Amouzadeh H R 1989 EffectsRhoads W S, Barton M H, Parks A H 1999 Comparison of of misoprostol and omeprazole on basal gastric pH and medical and surgical treatment for impaction of the free acid content in horses. Research in Veterinary small colon in horses: 84 cases (1986-1996). Journal of Science 47:350--354 the American Veterinary Medical Association Scarff K L, Judd L M, Toh B H et al1999 Gastric H(+), 214:1042-1047 K(+ )-adenosine triphosphatase beta subunit is requiredRichards W G, Sugarbaker 0 J 1995 Neuronal control of for normal function, development and membrane esophageal function. Chest Surgery Clinics of North structure of mouse parietal cells. Gastroenterology America 5:157-171 117:605-618
  • 121. 120 EQUINE CLINICAL PHARMACOLOGYSchoenfeld P, Kimmey M B, Scheiman J et al 1999 Steel C M, Bolton J R, Preechagoon Y et al1999 Unreliable Nonsteroidal anti-inflammatory drug-associated rectal absorption of cisapride in horses. Equine gastrointestinal complications: guidelines for prevention Veterinary Journal 31:82-84 and treatment. Alimentary Pharmacology and Thillainayagam A V, Hunt J B, Farthing M J 1998 Therapeutics 13:1273-1285 Enhancing clinical efficacy of oral rehydration therapy:Schumacher J, DeGraves F J, Spano J S 1997 Clinical is low osmolality the key? Gastroenterology and clinicopathologic effects of large doses of raw 114:197-210 linseed oil as compared to mineral oil in healthy horses. Thompson L P, Burrow J A, Madison J R et al 1994 Effect Journal of Veterinary Internal Medicine 11:296-299 of bethanechol on equine gastric motility and secretion.Schusser G E, White N A 1997 Morphologic and In: Proceedings of the 5th Equine Colic Research quantitative evaluation of the myenteric plexuses and Symposium, Athens, GA, p. 12 neurons in the large colon of horses. Journal of the Tillman L A, Drake F M, Dixon J S et al 1996 Safety American Veterinary Medical Association 210:928-934 of bismuth in the treatment of gastrointestinalSellers A F, Lowe J E, Rendano V T et al 1982 The reservoir diseases. Alimentary Pharmacology and Therapeutics function of the equine cecum and ventral large colon: its 10:459-467 relation to chronic non-surgical obstructive disease with Tonini M 1996 Recent advances in the pharmacology of colic. Cornell Veterinarian 72:233-241 gastrointestinal prokinetics. Pharmacological ResearchSjoqvist A, Hallerback B, Glise H 1985 Reflex adrenergic 33:217-226 inhibition of colonic motility in anesthetized rat caused Udassin R, Eimerl D, Schiffman J et al 1994 Epidural by nociceptive stimuli of peritoneum, Digestive anesthesia accelerates the recovery of postischemic Diseases and Sciences 30:749-754 bowel motility in the rat. Anesthesiology 80:832-836Smyth G B, Duran S, Ravis Wet al1990 Pharmacokinetic Valk N, Doherty T J, Blackford J T et al 1998 Effect of studies of cimetidine hydrochloride in adult horses. cisapride on gastric emptying in horses following Equine Veterinary Journal 22:48-50 endotoxin treatment. Equine Veterinary JournalSojka J E, Adams S B, Lamar C H et al 1988 Effect of 30:344-348 butorphanol, pentazocine, meperidine, or Vitola J V, Forman M B, Holsinger J P et al 1997 Reduction metoclopramide on intestinal motility in female ponies. of myocardial infarct size in rabbits and inhibition of American Journal of Veterinary Research 49:527-529 activation of rabbit and human neutrophils by lidocaine.Sojka J E, Weiss J S, Samuels M L et al 1992 Effect of the American Heart Journal 133:315-322 somatostatin analog octreotide on gastric fluid pH in Washabau R J, Hall J A 1995 Cisapride. Journal of ponies. American Journal of Veterinary Research the American Veterinary Medical Association 53:1818-1821 207:1285-1288Sondheimer J M, Arnold G L 1986 Early effects of Watson T D, Sullivan M 1991 Effects of detomidine on bethanechol on the esophageal motor function of equine oesophageal function as studied by contrast infants with gastroesophageal reflux. Journal of radiography. Veterinary Record 129:67-69 Pediatric Gastroenterology and Nutrition 5:47-51 Zhao X T, Wang L, Lin H C 2000 Slowing of intestinalSteel C M, Bolton J R, Preechagoon Y et al1998 transit by fat depends on naloxone-blockable Pharmacokinetics of cisapride in the horse. Journal efferent, opioid pathway. American Journal of of Veterinary Pharmacology and Therapeutics Physiology: Gastrointestinal and Liver Physiology 21:433-436 278:G866-G870
  • 122. CHAPTER CONTENTSIntroduction 121Anti-inflammatory drugs 121 Intraarticular medication Corticosteroids 121 Hyaluronan 125 Dimethyl sulfoxide 126 Tom YarbroughLocal anesthetics 127Antimicrobial agents 128Synovectomy with radiopharmaceuticals 130References 131 INTRODUCTION Selection of intraarticular medication is a daily process for the equine practitioner. This chapter presents some of the literature that exists on the individual drugs described. Wherever possible, studies using horses have been selected to reduce the bias created by species variation and topics have not been included if there is no real consensus opinion; it is hoped that this will present what can be considered as the "industry" accepted opinions. ANTI-INFLAMMATORY DRUGS Many drugs that have an anti-inflammatory action may also have other mechanisms of action. Corti- costeroids are one of the most commonly adminis- tered classes of intraarticular medication in equine medicine. Hyaluronan (sodium hyaluronate, HA) is a natural component of the joint and its intra- articular use has anti-inflammatory actions and may have lubricating effects. Similarly dimethyl sulfoxide is used for both its anti-inflammatory and its antimicrobial actions. CORTICOSTEROIDS Safe and effective use of corticosteroids requires at least a cursory understanding of their unique pharmacology and an understanding of the under- lying disease process. Corticosteroids are a family of molecules each containing 21 carbon molecules distributed between one 3-carbon and three 6-carbon rings (Axelrod 1993). It should be 121
  • 123. 122 EQUINE CLINICAL PHARMACOLOGYrecognized that all corticosteroids are not effec- the receptor, a conformational change in thetive when used intraarticularly. Cortisone and receptor allows the construct to bind directly toprednisone would be ineffective since they must DNA, subsequently modulating gene expressionbe converted in the liver from the inactive ll-keto (Cooper 1997,Schimmer & Parker 1996).form to the active 16-betahydroxyl form (Axelrod The potential mechanisms by which cortico-1993). Within the corticosteroids, various modifi- steroids might alter the homeostasis of the jointcations of the basic molecule are utilized to modify space proper are numerous. Corticosteroids maythe duration of activity. The most highly water- benefit any inflammatory condition by modulat-soluble phosphate and succinate esters are consid- ing the influx of inflammatory cells into the targetered the most rapid in onset and have the shortest region. This effect is at least in part a result of theirduration. Duration is felt to be longer in the acetate ability to reduce capillary permeability, reduceand acetonide substituted forms, followed by the leukocyte chemotaxis and adhesion and decreasehexacetonide ester, which is felt to impart the vascular permeability. In addition to modulatinglongest duration of action. Structural modification the numbers of cells within the region of inflamma-of the ring structure is only one factor in determin- tion, corticosteroids also have the ability to modifying the duration of action of particular cortico- the cellular responses as well. Neutrophil functionsteroids. Carrier formulations and hydrolysis can can be ameliorated by reducing phagocytosis andeach subsequently affect the absorption and acti- decreasing lysosomal enzyme and prostaglandinvation of these drugs. Although many of the tests release. The most "body wide" effect is created byused to determine efficacy and duration of action membrane stabilization and reduced formation ofdo not take into account variations present in phospholipase A z, with the subsequent reductionintraarticular use, it still useful for the clinician to in prostaglandin production in a wide variety ofhave an understanding of the relative potency and cells (Axelrod 1993, Boumpas et al 1993). Some ofduration of action of many of the more commonly the more pertinent research information surround-used corticosteroid formulations (Table 7.1). ing the use of corticosteroids in the joint is pre- Corticosteroids affect cells through a well- sented to aid in establishing an opinion as to thestudied series of interactions. All corticosteroid benefit and timing of corticosteroid use.hormones basically act by diffusing through thecell membrane and interacting within the cyto-plasm with a specific set of proteins in the corti- Methylprednisolone acetatecosteroid receptor superfamily. This is a large class Methylprednisolone acetate (MPA) is a syntheticof receptors that contains highly related domains corticosteroid produced as the 6a-methyl deriva-for ligand binding and resultant transcriptional tive of prednisolone. Pharmacological studies ofactivation. Once the corticosteroid has bound to the esters of MPA show that it is rapidly converted to methylprednisolone, the active form. Reversible metabolism of methylprednisolone to methyl- Table 7.1 Anti-Inflammatory potency and duration of action of corticosteroids used commonly for prednisone creates the inactive metabolite. The intraarticular administration conversion from the prodrug to the active form of the drug is partially responsible for variations Compound Anti-inflammatory Duration of identified in the classification of the duration of potential action (h) MPA activity. High levels of the active product, (cortisol = 1) methylprednisolone, have been identified in syno- Cortisone 0.8 8-12 vial fluid within 2 h of MPA injection. Despite the 60 -Methylprednisolone 5 12-36 persistence of MPA levels for 5-39 days after Triamcinolone 5 12-36 injection, the active metabolite was only identifi- Betamethasone 25 36-72 able for 2-6 days (Auteflage et al1986). The potential beneficial and detrimental effects Dexamethasone 25 36-72 of MPA on articular cartilage and chondrocytes
  • 124. 7. INTRAARTICULAR MEDICATION 123 have been studied in many different systems. Cell With these systems, we have the ability to assess culture systems offer an easy cost-effective means the presence of a time-averaged stable matrix, the to assess some of the basic effects of corticosteroids sine qua non of healthy articular cartilage. Some on cartilage. The primary problem is that chondro- of the conflicting realities of the complete bio- cytes devoid of their normal matrix and sheltered logical system were demonstrated in an osteo- from their constant biomechanical stimulation do chondral fragment-induced model of middle not perfectly mimic the in vivo state. Maintenance carpal arthritis/synovitis. The use of MPA, ver- of much of the extracellular matrix in cultured sus a polyionic fluid control, resulted in reduced explants is a step towards reconstituting the influ- prostaglandin E2 concentration in the synovial ence of the intercellular nanostructure, but these fluid, improved microscopic scores for intimal systems also vary greatly from what is seen in vivo. hyperplasia and improved vascularity in syno- Explant samples remain metabolically active and vial membrane. However, the occurrence of viable but gradually lose proteoglycans. This increased articular cartilage erosion and morpho- gradual loss of proteoglycans and a concomitant logical lesions suggested that the combination of reduction in proteoglycan synthesis have been work and corticosteroids was detrimental indiscovered after administration of MPA (Jolly active synovitis (Frisbie et aI1998). It should alsoet al 1995). It was shown that the aggrecan that is be noted that the model here required that the produced was reduced in size and demonstrated animals be exercised with the osteochondralincreased polydispersity. In the same study, the fragment still present in the joint space, a condi-investigators discovered that MPA induced the tion most of us would avoid whenever possible.synthesis of small non-aggregating proteogly- Suggestions have been made that the conco-cans. Both of these effects can result in a molecule mitant administration of HA might be able towith a reduced ability to interact with HA, thus ameliorate some of the detrimental effects ofreducing proteoglycan retention in the matrix corticosteroids on proteoglycan formation and(Todhunter et aI1996). Reduction in the ability of release (Tulamo 1991). A single intraarticulararticular cartilage alone would be considered a rea- injection of MPA (50 mg) has been shown toson for not using corticosteroids if this were the only decrease type II procollagen and aggrecanfactor. Some beneficial effects have been implied mRNA (MacLeod et al 1998). Further evidencefrom studies demonstrating a MPA-induced of the suppressive effects was seen in ponies,reduction in matrix metalloproteinase (MMP) 13 where it was shown that MPA-treated joints hadproduction following stimulation with recombi- significantly lower glycosaminoglycan contentnant human interleukin 1 (IL-1) (Caron et aI1996). in the articular cartilage than control ponies.Studies assessing the effects on stromelysin, This response was not ameliorated by the sys-probably the most detrimental MMP, have been temic administration of polysulfated glycos-less promising (May et al 1988). The general aminoglycans (PSGAGs) (Fubini et al 1993). In animpression with most of these systems is that the instability model in dogs, created by transectionanticatabolic effects of MPA on the proteoglycan of the anterior cruciate, some overall beneficialcontent of the explants far outweigh any antiana- effects were seen. MPA was shown to reduce thebolic or cytotoxic drug effect at clinically impor- size of osteophytes, the severity of histologicaltant doses (Jolly et al 1995). cartilage lesions and the production of the MMP Isolated systems are good for answering basic stromelysins (Pelletier et al 1994). The ability toscientific questions but are often difficult to extrap- reduce inflammatory mediators and cartilageolate to clinical settings, especially when assessing degeneration is of paramount importance inthe response of cartilage to the effects of cortico- arthritic conditions. Since most of our patientssteroids without the influences of the synovium. are not suffering from an instability-inducedIn vivo modeling allows the study of more clini- arthritis and we generally have an articularcally relevant situations and reduces the pheno- component to the disease, the ability to eliminatetypic changes present with cell culture systems. the inflammatory mediators in conditions of
  • 125. 124 EQUINE CLINICAL PHARMACOLOGYtraumatic synovitis/arthritis must be weighed IL-1a in the presence of concomitant human plas-up against the effects on the healing of cartilage minogen, glucocorticosteroids statistically sig-defects. The use of MPA in the immediate post- nificantly inhibited catabolism. The authors feltoperative period has been brought into question that the inhibition of cartilage degradation by theby evidence suggesting that it reduces the quan- glucocorticosteroids might occur through down-tity and quality of healing tissue in experimentally regulation of urokinase plasminogen activatorinduced cartilage defects (Carter et aI1996). (u-PA) activity (Augustine & Oleksyszyn 1997). Other suggested benefits of MPA treatment Unlike the results described for MPA, triam-include improving joint health by increasing lubri- cinolone acetonide used in another model ofcation in the joint. Injection of 100mg into normal exercising horses with carpal osteochondralcarpi has been shown to increase the levels of fragment-induced arthritis resulted in signifi-surface-active phospholipid in the joint, resulting cantly less lameness, lower total protein, higherin improved cartilage hydration, promotion of HA and glycosaminoglycan concentrations inmacrophage activity and the ability to scavenge synovial fluid, less inflammatory cell infiltration,oxygen free radicals (Hills et aI1998). subintimal hyperplasia and subintimal fibrosis in the synovial layer, and improved articular carti-Triamcinolone lage histomorphological parameters (Frisbie et al 1997). Similar results were found in dogs withMany of the studies designed to assess cortico- transection of the anterior cruciate. Triamcinolonesteroids ability to affect the anabolic and catabolic significantly reduced the size of osteophytes andeffects of inflammatory mediators have been car- the histologic severity of cartilage lesions on theried out with triamcinolone. In explant cultures, condyles. Microscopic evidence of improved car-recombinant IL-la has been shown to increase tilage health included a reduced percentage ofdegradation and reduce the synthesis of proteo- immunoreactive chondrocytes for stromelysin,glycans, as did triamcinolone. The combination c-Fos and c-Myc (Pelletier et aI1995).of human recombinant insulin-like growth factor 1(IGF-l) and corticosteroids was able to normalize Betamethasonecollagen production and proteoglycan catabolismand anabolism in this system (Frisbie & Nixon High doses of intraarticular betamethasone in1997). In a murine model using normal cartilage, rabbits have been associated with cellular degen-IGF-l resulted in significant enhancement of eration (Papachristou et aI1997). In a study usingchondrocyte proteoglycan synthesis. In arthritic more clinically relevant doses, the chondropro-cartilage, IGF-1 failed to stimulate proteoglycan tective effect of betamethasone was examined tosynthesis and only proteoglycans with relatively determine if it could decrease articular cartilagesmall dimensions were produced in the presence injury caused by Staphylococcus aureus gonioarthri-of the corticosteroid triamcinolone acetonide tis. Rabbits that received antimicrobial treatment(Verschure et al 1994). Interestingly, in murine plus parenteral betamethasone demonstrated sig-patellar cartilage, triamcinolone in combination nificantly less articular cartilage proteoglycan losswith IGF-1 was able to stimulate proteoglycan than rabbits treated with antimicrobial agentssynthesis and maintain the synthesis of hydrody- alone. Intraarticular betamethasone was some-namically large proteoglycans by chondrocytes what less effective in this regard, possibly reflect-(van der Kraan et aI1993). The ability of triamcin- ing the smaller corticosteroid dosage (Strickerolone to protect cartilage against the effects of et aI1996).IL-l-induced catabolism has been shown to vary Any time a needle is placed into the joint, a pos-with the inciting cause in bovine cartilage explants. sibility exists for infection. The most commonlyWhen the degradation was induced by IL-la isolated organism following injection is Staph.alone, triamcinolone showed no protective effects; aureus (Schneider et al 1992). There is evidence tohowever, when the degradation was induced by suggest that the use of intraarticular corticosteroids
  • 126. 7. INTRAARTICULAR MEDICATION 125 reduces the numbers of bacteria required to owner compliance, then the clinician should induce infection (Gustafson et al 1989). A subse- select a corticosteroid that has as short a duration quent study suggested that these effects could be of action as possible and that produces minimal blocked by the addition of an aminoglycoside effects on the cartilage. antibiotic to the medication. However, clinicalexperience would indicate that good patient preparation and situation control provide enough HYALURONAN protection against this complication. Depending HA is the most widely used intraarticular medica-on the organism involved, the corticosteroid tion in horses. The selection of this medicationinjected and the underlying process being treated, and the route by which it is administered is oftensigns of infection may not be evident for some decided upon without consideration of the under- time in corticosteroid-treated animals. Just to be lying disease process or what is being asked ofsafe, owners should be instructed to watch for the drug itself. Naturally occurring HA is a rela-any signs of heat, swelling or increased lameness tively ubiquitous molecule in mammals. It is pro-for at least 2 weeks after the injection. duced naturally by a membrane-bound enzyme The literature on intraarticular corticosteroids HA synthase. It is often quoted that the HA pres-might seem to hold no middle ground for the cli- ent in synovial fluid is from the fibroblastic syno-nician, but careful assessment helps to identify a viocytes. Recently, it has been demonstratedlogical path for treatment. Ideally, corticosteroids that there are at least three isoforms of HA syn-should only be used in low-motion joints, in thase and that the enzymes are expressed in cellhorses that have had the inciting lesion corrected culture by synovial cells, chondrocytes andsurgically or in animals that can be rested during osteosarcoma cell lines (Recklies et al 2001). HAthe period that the corticosteroids could have itself is a relatively simple molecule of repeatingreduced the proteoglycan content. It is generally disaccharide units that performs some fairlyconsidered ineffective simply to medicate away amazing effects once extruded into the extracellu-intraarticular fragmentation. With the accessibil- lar matrix. It has the ability to confer extraordinaryity and reliability of arthroscopy, it is generally compressive strength to the articular cartilageconsidered a second option to continue pushing when functioning as the core molecule for pro-any athlete along by palliating the effusion asso- teoglycan aggregates (Ratcliffe & Mow 1996). Itciated with articular fragments. In these cases, likewise imparts the viscoelastic nature to thewhere the owners opt not to remove the offend- synovial fluid (de Smedt et a11993, Ribitsch et aling lesion and simply remove as much of the 1999). With regard to whatever role HA plays ininflammatory process as possible, the use of HA lubrication of the joint surfaces, there is somein conjunction with the corticosteroids might be question as to whether the molecular weight ofuseful in reducing the rate of deterioration. This the molecule has an effect on its action (Kato et alis especially important in high-motion joints. 1995, Mabuchi et aI1999).Likewise, it is logical to reach for other methods While the actions of naturally occurring HAof reducing inflammation, such as non-steroidal are fairly well characterized, the beneficial effectsanti-inflammatory drugs (NSAIDs, see Ch. 14), of the exogenous products are very elusive. In aintra muscular (i.m.) PSGAG, capsaicin and clinical setting, we often see prolonged benefit inphysical therapy methods rather than using fre- the form of reduced effusion and reduced severityquent injections. It can be seen that some degree of lameness for prolonged periods. This is veryof reduced formation and aggregation of proteo- unusual for a product that has a half-life in theglycans is to be expected with administration of synovial space in the order of hours. In an acutecorticosteroids. Whenever possible, this response inflammatory process, it is thought that at leastcan be minimized by resting the animal after injec- one possible beneficial mechanism is the returntion to allow for return of normal levels of aggre- of the steric barrier provided by HA. In support ofcan in the matrix. When rest is again limited by this idea, HA has been shown to have a protective
  • 127. 126 EQUINE CLINICAL PHARMACOLOGYeffect against the invasion of polymorphonuclear decreasing all of the subsequent steps in thecells (Partsch et aI1989). Similar effects have been inflammatory cascade. This means that a definedseen with respect to lymphocyte proliferation and condition of pure synovitis would be the mostmigration. In both cell types, it was felt that the ideal situation. In a clinical setting, this is rarely thehigher molecular weight forms were required to case, since most of the conditions we treat in theappreciate any benefit (Peluso et al 1990). The equine athlete involve some degree of underlyingactual mechanisms of this effect are yet to be elu- degenerative joint disease. In these conditions, wecidated but it is likely that it is related to cellular might still be able to reduce the rate of degenera-interactions with the CD44 molecule (Takeshita tion without contributing to the catabolic processet aI1997). Other possible anti-inflammatory ben- already underway in the articular cartilage. Itefits have been difficult to identify. The purported should be noted that HA has little ability to elimi-induction of endogenous HA and reduction in nate severe forms of lameness and might even haveMMPs have been refuted by some studies (Clegg deleterious effects when put into a joint whereet al1998, Lynch et aI1998). It does seem, however, inflammation is so overwhelming that the HA isthat the intraarticular use of both HA and PSGAG rapidly broken down to its low-molecular-weightreduces inflammation by modulating the produc- fragments (Peloso et aI1993). When using the med-tion of prostaglandin E (Frean & Lees 2000). ication as a protective in conjunction with cortico- Extensive research has been undertaken to steroids, it may be possible to select one of the lowerdetermine the possible anabolic and anticatabolic cost low-molecular-weight forms. Most cliniciansbenefits of HA on articular cartilage. As mentioned will opt to use the highest-molecular-weight formin the section on corticosteroid use, each study available when using it as a stand-alone therapy,must be weighed in the context of how closely it although this practice has recently again beenmimics clinical situations. HA has been shown to brought into question (Aviad & Houpt 1994).have a protective effect on cartilage degenerationin a cast confinement model. Repeated injections DIMETHYL SULFOXIDEover a 92 day period were shown to stabilize thecatabolic processes associated with atrophy of Dimethyl sulfoxide (DMSO) is a clear viscous liq-articular cartilage. It was felt in this situation that uid originally formulated as a solvent. It has gaineddownregulation of tumor necrosis factor (TNF) Q some medical use as an anti-inflammatory andproduced the chondrostabilizing influence on antimicrobial agent. Its anti-inflammatory prop-articular cartilage (Comer et al 1996). HA has erties have been primarily attributed to its abilitybeen shown to stimulated prostaglandin synthe- to scavenge free radicals (Fox & Fox 1983). Thesis in both cell and explant cultures (Frean et al potential detrimental effects of DMSO have been1999). Mixed results have been obtained when studied in both cell culture and in vivo systemsusing explant cultures for examination of the pro- (Moses et al 2001). DMSO has been shown totective effects of HA. In one study, its use was have a deleterious effect on the health of synovialshown to be beneficial in reducing prostaglandin membrane cultures; however there were norelease induced by human recombinant IL-1f3 in changes observed in chondrocyte viability aftermetacarpal hyaline cartilage, while it had no effect explant incubation with DMSO. The mild effector appeared to increase prostaglandin release in on proteoglycan metabolism seen in anotherfibrocartilage and navicular hyaline cartilage study was found to be only transient (Smith et al(Frean et al 2000). 2000). In horses, injections of 2 ml of a 40% solu- When choosing HA as a therapeutic agent, it is tion of DMSO in lactated Ringers solution for abest to have at least some underlying sense of the total of three injections caused no evidence ofcondition in question. Ideally it is best suited to cartilage degradation and no alterations in gly-acute inflammatory conditions where its ability cosaminoglycan content (Welch et aI1989).to reduce the influx of white blood cells into the Recently, the use of various concentrations ofjoint proper could modify the disease process by DMSO in balanced polyionic fluids has been
  • 128. 7. INTRAARTICULAR MEDICATION 127advocated for reducing the inflammatory process fall in pH occurs, as in sepsis, the effect may bein the joint (Schneider et aI1992). It has been used completely blocked. It is possible to counter thisin concentrations from 5 to 40% in both traumatic to some degree by adding sodium bicarbonate toand septic arthritis. Clinically, the use of DMSO the locale prior to local anesthetic administration.in lavage fluids appears to result in reduced edema More highly bound anesthetics are less availableformation, increased range of motion and better for diffusion through the cell membrane. The netmaintenance of arthrotomy portals in conditions effect is a prolonged duration of action and aof septic arthritis. more prolonged onset of action. It has been shown that injection of lidocaine (lignocaine) or mepivacaine hydrochloride intoLOCAL ANESTHETICS the equine middle carpal joint increases synovial fluid cellularity. Repeated arthrocentesis by itselfIntraarticular analgesia is commonly used in the caused a moderate increase in cell counts, whilediagnosis of lameness, to enable other invasive injection of local anesthetics caused a greaterprocedures involving the joint and as a means of increase. Alterations in mucin clot quality, HA con-postoperative pain relief. Most commonly used tent, fluid viscosity, total protein and immunoglob-local anesthetics are weak base amide solutions. ulin G were generally of no significance (KirkhamA cursory understanding of the mechanisms of et aI1999). In this study, clear differences betweenaction of these agents is useful when assessing responses to the drugs could not be identifiedtheir effects. The onset and duration of action (White et al 1989). An older study determinedexpected from each drug is a product of their that mepivacaine was less reactive than lidocainemolecular weight, pH, lipid solubility and protein when assessing synovial fluid parameters (Spechtbinding. Diffusion across the cell membrane is et al 1988). The use of local anesthetics duringimportant in determining the onset of action, arthroscopy is a common procedure in veterinarysince diffusion is inversely related to the square and human orthopedics, although evidence thatroot of the molecular weight of the molecule. As this is of benefit over NSAIDs is lacking (Sorensenshown in Table 7.2, the molecular weight of the et aI1991).commonly used drugs is so similar as to have aminimal effect within the group. In the body,where the amide local anesthetics exist both in an Morphineionized and a nonionized form, the distribution During the 1990s, preclinical investigations inis a reflection of the pH of the surrounding tissue. models showed that local injection of smallAs the tissue pH falls, more of the drug is in its doses of opioid analgesics at sites of inflammationionized form and, since the nonionized portion is produce potent analgesia (Stein et al 1996).the form that has the ability to diffuse through the Interestingly, these same low doses given systemi-cell membrane to cause blockade of the sodium cally or into the uninflamed site are without effectchannels, efficacyis reduced. If a significant enough (Sibinga & Goldstein 1988). Intraarticular opioids have antinociceptive and anti-inflammatory actions in animals (Barber et al 1990, Kolesnikov Table 7.2 Features of some commonly used local anesthetics et al 1996). In humans, intraarticular morphine produces potent inhibition of acute postoperative Agent Molecular pK a Protein pain after knee surgery (Kalso et al 1997, Stein & weight binding (%) Yassouridis 1997). Both the analgesic and the anti- Lidocaine 236 7.9 64 inflammatory effects are apparently mediated by (lignocaine) peripheral opioid receptors, which have been Mepivacaine 246 7.6 77 identified on peripheral sensory nerve terminals Bupivacaine 288 8.1 96 (Stein et al 1996). Synthesized in the dorsal root ganglia, these receptors are axonally transported
  • 129. 128 EQUINE CLINICAL PHARMACOLOGYtowards the nerve terminals, where they can be adequate preparation and situation control prioractivated by exogenous agonists as well as by to injection. Whenever possible, the selection ofendogenous opioid peptides expressed in inflam- the antibiotic should be based on culture andmatory cells (Khoury et al 1994). Local opioid sensitivity results (see Ch. 2). In situations whereanalgesic effects are more pronounced in inflamed this is impossible, the clinician should select forthan in non-inflamed tissue. This may be related the narrowest possible spectrum against the mostto an upregulation of peripheral opioid recep- likely organisms.tors, to their enhanced coupling with G proteins The two antimicrobial agents used most com-or disruption of the perineural barrier, leading monly intraarticularly are gentamicin andto an improved access for opioid agonists (Shah amikacin. Gentamicin offers many advantageset al 1997). Analgesia provided by the binding from the standpoint of reaching high synovialto the opioid receptors is further enhanced by fluid concentrations with doses as low as 150mgthe morphine-induced reduction in substance without inducing significant synovitis (Lloyd et alP release from the nociceptive nerve terminals. 1988a). At this dose, peak concentrations in syno- Postoperative analgesia from morphine has vial fluid of 1828 flg/ml can be achieved versusbeen shown to be the most effective if administered 2.53 flg/ml with systemic therapy (Lloyd et alat the completion of the procedure (Brandsson et al 1988b). The potential to achieve such high levels2000, Reuben et al 2001, Tetzlaff et al 2000). In within the joint allows the clinician to use anthese cases, it appears that the postoperative use antimicrobial agent that might have been seen asof morphine allows the clinician to reduce both moderately effective in the laboratory based solelythe level and the duration of other analgesics. on the minimum inhibitory concentration (MIC)This is not to say that the only potential benefit results. Other considerations when selecting anof morphine is in the postoperative patient. antimicrobial agent for intraarticular use shouldMorphine has also been shown to be of equivalent include an understanding of the physical proper-effect to corticosteroid administration in other ties of the antimicrobial agent, its mechanismsforms of chronic arthritides (Keates et al 1999, of action, half-life in synovial fluid and potentialStein et al 1999). The reductions in inflammatory effects on the synovial membrane and articularcell influx, reduced edema formation and analge- cartilage. If possible, a complete synovial fluidsia provided with minimal systemic effects make analysis, including pH, cell count and total protein,intraarticular morphine a very attractive postoper- is useful in establishing the possible environmen-ative therapy. I most commonly use a combination tal effects of the fluid on the efficacy of theof 5-15 mg morphine with 6 mg lidocaine for antimicrobial agent. The various environmentalpostoperative analgesia and have seen no unto- conditions likely to be encountered at a site ofward effects. The beneficial effects with respect to infection have been evaluated for their effect onimproved analgesia and ability to reduce the antimicrobial agents. The MIC of gentamicin andusage of NSAIDs remains to be proven. amikacin were increased up to five-fold at pH < 6.5. Likewise, magnesium and calcium ion concen- trations > 10mmol/I and ferric iron concentrations of 10mmol/l increased aminoglycoside MIe valuesANTIMICROBIAL AGENTS from 3.66- to 8-fold (Nanavaty et al 1998). These results demonstrate the benefit of complete fluidIntraarticular agents analysis or a good overall knowledge of pathol-Antimicrobial agents are used routinely intra- ogy induced by the inciting agents.articularly both for the treatment of established Other antimicrobial agents that have been usedinfections and as a preventive measure during intraarticularly include methicillin, ticarcillin-administration of other therapeutic and diagnos- clavulanic acid, ceftiofur and imipenem. Ceftiofurtic medications. Obviously, the use of antibiotics has been shown to develop high synovialshould never be considered as a substitute for fluid concentrations following intraarticular
  • 130. 7. INTRAARTICULAR MEDICATION 129administration. Following administration of form the beads until the cement is no longer150mg into the antebrachiocarpal joint, levels adherent to latex gloves when touched.peaked at 5825.08f.Lg/ ml at 15 min, compared The aminoglycosides amikacin and gentamicinwith 1.43f.Lg/ml after i.v. administration. The half- are the most commonly studied antimicrobiallife in these normal joints was 5.1h. Assuming that agents for use in PMMA. The studies have lookedthese results correlate with what would be encoun- at a number of questions including the possibilitytered in an infectious condition, MICs for most of altered antimicrobial sensitivity in the presenceorganisms would be met with a single injection of PMMA, the elution rates and the ability toevery 24h (Mills et al 2000). Imipenem-cilastin incorporate the injectable versus the poweredusage once daily (500mg) for 3 days was forms into the cement (Arciola et al 2001,shown to have minimal inflammatory side-effects Ethell et al 2000). Unlike the ability to use intra-(Schneider 1999). articular injection of antimicrobial agents as a means of achieving high enough levels to over- come microorganisms with borderline sensitiv- ity, care should be taken to select antimicrobialSustained-release preparations agents described as "sensitive" when choosingThe use of sustained-release systems for antimi- one to incorporate into a carrier. Sensitivity tocrobial agent delivery offers many advantages to ampicillin, cefamandole, cefazolin, imipenem,the equine practitioner. The ability to target high, amikacin, netilmycin, erythromycin, trimetho-sustained levels of an antimicrobial agent at the prim-sulfamethoxazole, chloramphenicol andsite of infection allows us to utilize many antimi- vancomycin have all been shown to be reducedcrobial agents that would otherwise be prohib- following adhesion of bacteria to PMMA (Arciolaited based on cost or systemic side-effects. For a et al 2001). In vitro studies would suggest thatcarrier- antimicrobial agent composite to be either the powdered or the liquid form is appro-effective, it must be minimally reactive, provide priate, with sustained release for up to 30 daysconsistent elution properties and be easy to pre- following doses as low as 150 and 250mg per 2 gpare during procedures or store well if prepared cement. Clinically higher doses, 1-3g per 109in advance. The most commonly described car- cement, are used when maintaining the materialrier is polymethylmethacrylate radio opaque properties of the cement are not a concern.bone cement. An understanding of the physical Ceftiofur has been examined as a possibleproperties of both the antimicrobial agents and antimicrobial agent for use in PMMA deliverythe carrier are necessary to utilize this modality systems (Ethell et al 2000). Results demonstrateto its fullest potential. The elution rates of the that it is also an appropriate antimicrobial agentantimicrobial agent can be increased by using for combination with PMMA. Its elution profilehigher numbers of smaller beads and by making would suggest that it is released from the carrierthe beads as spherical as possible. These tech- rapidly and, therefore, should be placed in such aniques are designed to increase the overall sur- fashion that removal and replacement of expendedface area presented to the body since the elution beads is possible if prolonged drug levels are toof the antimicrobial agent is from the surface of be required.the beads. The technique described for prepara- Other antimicrobial agents that have been incor-tion of the beads involves mixing the antimicro- porated into PMMA beads and shown to havebial agent (powdered or liquid) with the useful elution profiles include penicillin G (ben-powered PMMA, thoroughly mixing the two and zylpenicillin), ampicillin, amoxicillin, flucloxacillin,then adding the liquid activator. If a mold is to be amoxicillin-clavulanate, cefamandole, cefazolin,used, the composite mixture should be injected tobramycin, netilmicin, imipenem, erythromycin,immediately and the molds formed. If the beads ciprofloxacin, trimethoprim-sulfamethoxazole,are to be hand fashioned, care should be taken chloramphenicol and vancomycin (Ethell et alduring the mixing process to avoid beginning to 2000, Mader et a11997, Veyries et aI2000).
  • 131. 130 EQUINE CLINICAL PHARMACOLOGY The use of biodegradable materials as vehicles maintained in the target region by blocking effluxis an appealing alternative. Plaster of Paris (POP), through the use of pneumatic tourniquets placeddemineralized bone matrix, polylactic acid (PLA) at the level of the joint proximad and distad to theand poly(dl-lactide)-coglycolide (PL:CG) have site of interest. The antimicrobial agent is furtherbeen examined for possible use (Mader et al1997, confined to the region of concern by leaving anMiclau et al 1993, Santschi 2000). The PLA and Esmarch to collapse the superficial vasculature.PL:CG biodegradable beads have been shown to The basic technique involves isolating a conve-release high concentrations of antimicrobial nient region for placing a cannulated screw intoagents in vitro for 4-8 weeks (Mader et al 1997). the cortex of the target bone or the palmar, plantarPlaster of Paris and demineralized bone matrix or palmar digital or plantar digital vein for catheterbeads elute antimicrobial agents at higher levels placement. Once the screw or catheter is placedover a much shorter period (Bowyer & Cumberland and hooked up to the antimicrobial agent delivery1994, Miclau et al 1993, Santschi 2000). Unlike system, the Esmarch is applied to the limb fromPMMA beads, which can be mixed, formed and distal to proximal. After the blood has beenused in a matter of minutes, POP beads require expressed from the vasculature, a pneumaticmore prolonged curing. The technique for prepar- tourniquet is inflated as described previously.ing beads from the powdered POP described by Selection of the appropriate antimicrobial agent,Santschi (2000) is as follows. Mix 5 ml gentamicin dose, volume and infusion rate are important.sulfate (100mg/ ml) or 4 ml amikacin (250mg/ ml) Sodium and potassium penicillin, ampicillin, gen-with 10 ml phosphate-buffered saline then add to tamicin and amikacin have all been used success-30g of calcium sulfate hemihydrate. Place the fully (Santschi 2000). Care should be taken whenbeads in a bead mold (with or without a suture selecting an antimicrobial agent not mentioned onleader) and cure overnight. Gas sterilize before this list. For instance, enrofloxacin will induce anuse (Todhunter et aI1996). extreme vasculitis following regional perfusion. Beads can be placed into joints or tendon sheaths Wide variations exist in the dose required for high-as an adjunct therapy in septic conditions. When pressure perfusion; the original descriptions calledused within a synovial structure, we most com- for using the typical systemic dose for each per-monly string the beads in series on non-absorbable fusate. Recent recommendations have cited dosessuture material to allow for removal at the end of as low as 250-500mg of amikacin and 100-300mgtreatment. There is little risk of using the beads of gentamicin for adult horses and 50 mg for foalsintraarticularly in most uniaxial, hinged joints in (Santschi 2000). For adult horses the antimicrobialthe horse since the sliding action of the apposing agent selected is diluted to a volume of 60 rnl (30rnlarticular surfaces is at little risk of damage during when perfusing the digit); in foals the volume cantimes of confinement. This would not be the case if be reduced to 10-12ml. Infusion rates vary withone were to attempt to use beads in the carpus, the individual clinicians preference. Most com-where the "tight packed" conformation is lost dur- monly, we use 2ml/min as a standard rate,ing flexion and extension. In these cases, we prefer reduced to 1 ml/rnin when perfusing the foot.to use a bandage cast to reduce the risk of the beadsbeing trapped between the two articular surfaces. SYNOVECTOMY WITH RADIOPHARMACEUTICALSRegional perfusionRegional perfusion was first assessed in the Activation of the synovial membrane has theequine literature as a technique for achieving potential to damage articular cartilage by thehigh levels of antimicrobial agents in the carpus liberation of inflammatory mediators and destruc-(Whitehair et aI1992). The basic principle involves tive enzymes and the recruitment of inflammatorythe saturation of tissues with volume-diluted cells into the joint (Goldberg & Toole 1987,antimicrobial agent. The antimicrobial agent is Lukoschek et al 1990). Common treatments for
  • 132. 7, INTRAARTICULAR MEDICATION 131synovitis are directed at reducing one or all of these damage. The degree of vascular damage, com-effects through rest, administration of steroidal bined with the tissues ability to revascularize, willand non-steroidal pharmaceuticals or immuno- be important in determining the degree of fibro-suppressive drugs, or by synovectomy techniques. sis in the area after it recovers from the radiation- Surgical synovectomy is technically difficult induced insult.and time consuming and requires general anesthe- In mature cartilage, chondrocytes rarely divide,sia. Consequently, less-invasive means of remov- providing them with a relative degree of resis-ing the synovial lining, including the intraarticular tance to radiation-induced damage. The principaladministration of agents, has been explored. factor in radiation-induced cartilage damage isCartilage damage and the pain associated with the damage to the fine vasculature of the peri-the administration of most chemical agents have chondrium, with the resultant death of the cellsmade radiation synovectomy an attractive alter- dependent on their nutritional supply. The lim-native (Shortkroff et al 1992, Zalutsky et al 1986, ited penetration of the carrier particles into theZuckerman et aI1988). subintimal tissues and the shallow penetration of Samarium-lS3 hydroxyapatite micro spheres the beta-radiation generated by the 153Sm makese S SmM) have been shown to ablate effectively 3 it unlikely that the fine vasculature of the peri-normal and inflamed equine synovium, with chondrium is at a significant risk of damage evenminimal radiation hazard to the horse or medical with repeated use of 153SmM.personnel (Yarbrough et al 2000a,b). Within the The use of 153SmM in the controlled environ-joint space, the synovial lining primarily absorbs ment of surgically induced arthritis appears tothe energy from the lS3Sm emissions, thus allowing provide another means of addressing the soft tis-the clinician to target the synovium for destruction. sue components of arthritis without any extracap-When 153Sm is combined with hydroxyapatite, sular leakage of radioactivity. Its proposed uses inexposure to other organs or support personnel is the horse include the management of preexistingminimized. proliferative synovial membranes in long-standing Within the immediate area of the diarthroidial arthritides, the removal of reactive synovia follow-joint, the endothelium and the synovial lining ing osteochondral fragment removal, treatmentcells are considered the most radiosensitive tis- of idiopathic synovitis/tenosynovitis and as asues. Therefore, radiation-induced synovectomy potential ancillary treatment in septic arthritis. Itmay produce beneficial effects by direct damage appears to provide a safe, effective means ofto the synovial lining cells, caused by the ioniza- removing the synovium without inducing histo-tion and excitation of the tissue atoms and the logical or histochemical damage to the articularformation of free radicals, induced by absorbed cartilage. Through customized variation in theradiation or by damage to the subintimal vascular carrier-isotope construct, the clinician can increasesupply, as described previously (johnson & Yanch or decrease the level of penetration and duration1991). The synovial intimal layer and the subinti- within the joint space to treat varying degrees ofmal vessels were two of the most dramatically synovial proliferation.affected cell populations in this study. Obstructionof capillaries may occur early in the process ofradiation-induced damage, through the swelling REFERENCESof the vascular endothelium or collagenous stroma Arciola C R, Donati M E, Montanaro L 2001 Adhesion to aor perivascular infiltrates. Later, fissuring of the polymeric biomaterial affects the antibiotic resistance oftunica media or loss of the endothelial cells may Staphylococcus epidermidis. Microbiologica (Pavia) 24:63-68result in the thrombosis of the affected vessels. Augustine A J, Oleksyszyn J 1997 GlucocorticosteroidsAlthough the actual sensitivity of the endothelial inhibit degradation in bovine cartilage explantscells to the effects of radiation is in question, it is stimulated with concomitant plasminogen and interleukin-1-alpha. Inflammation Research 46:60-64commonly considered to be one of the central Auteflage A. Alvineriere M, Toutain P 1986 Synovial fluidelements involved in radiation-induced tissue and plasma kinetics of methylprednisolone and
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